3D Gameplay's Multiple Character Conundrum

[Writer's Blah]

Just a little warning before we go ahead, this is going to be a pretty huge and in-depth post, so if you're gonna go ahead, I'd appreciate as much attention as you can muster. With that out of the way, let's dive in.

The Sonic fanbase, in all of its diversity, often wishes for things in the franchise to stray from the norm and attempt something which was done in the past. One of the most common requests I hear is a wish for multiple playable characters to be available in the next 3D Sonic game, after the 3D games have only had Sonic as the exclusive playable character since Sonic Unleashed in 2008. It's a reasonable request in my opinion, and one that SEGA should've been looking into for at least a decent while now. Disregarding the current lack of knowledge surrounding playable characters (or really, anything for that matter) in Project 2017, it seems that this is but a pipe dream for the time being. But really, I totally get wanting to be able to play as other characters in the Sonic franchise for a main series game again, but for a second, have you ever stopped to consider how that would even work? Let me explain what I mean.

The first game in the Sonic franchise to really do the whole multiple character thing was Sonic 3 & Knuckles. It's actually pretty surprising to note that the first game to attempt having multiple characters actually handled it extraordinarily well. See, from a mechanical perspective, there were only a few elements that separated one character's play experience from the others. Sonic had the insta-shield and secondary shield abilities, Tails could fly, and Knuckles could glide and climb vertical walls. It's worth noting that despite the level layouts never changing, the maps were sprawling enough that in situations where one character couldn't access a region (usually due to Sonic's inability to remove wall boulders or Knuckles' shorter jump height), there was always an alternate route to take where the character's natural abilities were instead allowed to shine. Knuckles' playthrough in particular is especially good at handling this, as the challenges in his playthrough focused on his gliding and climbing abilities instead of Sonic and Tails' more general speed and platforming challenges. It's through this focused level design where each character was able to feel like a new, fulfilling experience. But see, there was one other advantage that Sonic 3 & Knuckles had at play that made handling multiple characters a relatively smooth process: the fact that it was in 2D. Don't believe me?

When was the last time you played through Sonic 2 as Knuckles? The experience is an interesting one to say the least, but the main reason why this example is so fascinating to me is that Sonic 2 was never designed with Knuckles in mind. I played through this game recently, and as memory can recall, aside from one instance in Chemical Plant Zone, Act 1, which allows you to exploit the gliding and climbing skills to practically skip the entire level, the experience really isn't too different from vanilla Sonic 2. Sure, some parts are made slightly easier because of Knuckles' abilities, and some bosses are variably more challenging due to Knuckles' decreased jump height, but for the most part, you're really not going to get a radically different experience while playing as Knuckles. For further proof, take a look at the Knuckles in Sonic 1 hack, or heck, even the Sonic Advance trilogy handles character abilities within a pretty safe context. The point I'm trying to make is that in 2D, character abilities never feel overpowered because of the constraint of only dealing with two axes. To highlight why specifically, you need to study the way in which 2D levels, on the whole, are constructed.

Let's talk about how Tails' and Knuckles' abilities function within 2D level sensibilities. What both characters have at their disposal that Sonic lacks is control over the Y axis. In order to gain any kind of height, Sonic needs a sizable amount of speed and momentum and an upwards incline to launch Sonic into the air. This is the focal point of Sonic's speed in 2D, and what really separates him from his platforming contemporaries. The usage of speed isn't just a cool gimmick, it's integral to even reaching certain parts of the level, as the Genesis games' level design often highlights. However, Tails and Knuckles take this concept and run with it. Their ability to reach these higher paths can be done through Sonic's fashion, but they're also supplanted by their own natural abilities. Tails' flight is resource-based, as you can practically go anywhere you want in the level so long as you get there before Tails tuckers out. But his flight is incredibly flexible in that you can use it to travel a great horizontal distance, exclusively upwards, or any combination of the two. I often find myself using it to correct mistakes in my jumps where I would otherwise fall as Sonic or Knuckles. In comparison, Knuckles' abilities are more situational, but also have the boon of being unlimited, unlike Tails. If you need to clear a large gap, you can simply glide across at your own leisure. If you need to reach a higher part of the level, you can just climb a wall that lets you do just that. You can even combine both abilities to clear especially large chasms. These abilities really facilitate exploration, and given the Genesis games' focus on making levels that are large and multi-branching, these abilities are an excellent way to enhance the player experience.

But then, everything changed when the 3D nation attacked. With the release of Sonic Adventure on Dreamcast, three more characters were added into the spotlight, with the original three still present. But something many critics of the Adventure games are quick to jump on is how the multiple characters were handled. See, SA1 introduced the concept of having different playstyles and mission structures depending on the character you chose, and many people fail to see the logic in such decision. Now, instead of simply reaching the end of the level like Sonic, Tails' levels focused on racing Sonic to the end of the level, and Knuckles' levels were focused on finding emerald shards in more open-ended levels. Coming into it from a casual perspective, it seems weird, doesn't it? Why would SEGA make the decision to radically change the mission structure simply by which character you were playing as? Wouldn't it have been easier, not to mention somewhat more cost effective, to simply have Tails and Knuckles reach the end of levels like Sonic? Unfortunately, the answer isn't quite so simple.

As you can see here, Knuckles' glide has very different implications when applied to 3D space. What was just a helpful ability to help make exploring easier now becomes an overpowered move that basically invalidates half of the level design. The addition of the Z axis makes designing levels with Tails and Knuckles in mind significantly harder. Because of how open 3D level design is by nature, in comparison to 2D level design, questions such as "how will these characters' iconic abilities affect the gameplay" are very big, important questions. If you think about it, Tails' and Knuckles' mission objectives make a lot more sense given this context. Tails' ability to fly make getting through levels much easier, but due to flight still being resource-based, it isn't impossible to still adapt Tails to more linear levels. Knuckles on the other hand, isn't so lucky. His unlimited gliding and wall-climbing abilities make creating a linear level for Knuckles very complicated, as you constantly have to account for making sure the player isn't able to sequence break so easily. Sonic Team decided to take the easy way out by simply designing more open-ended levels for Knuckles. The open nature of the levels makes sequence-breaking a non-issue, as now its more about using your abilities to traverse a box that's meant to be navigated around efficiently from all directions with the purpose of finding mini-objectives rather than a single, ultimate one.

But granted, I can still understand why one might wish for more linear level design. After all, finding emerald shards isn't everybody's cup of tea, and from a gameplay perspective, is enough of a deviation to feel alien to the core focus of a Sonic game. So barring Tails for the moment, are there any examples of more linear level design that accounts for Knuckles' abilities? Interestingly, there are, with one example in Sonic Adventure 2 (Cannon's Core), and multiple from Sonic 06, of all games. (Flame Core and Aquatic Base both have Knuckles segments, and Rouge has the entirety of Tropical Jungle, plus segments in White Acropolis and Kingdom Valley, not to mention the End of the World segment) What a good lot of these examples have in common however, is that they usually have to do with finding a certain number of items or activating a certain number of devices. The ones that play more with linearity (Tropical Jungle, White Acropolis, EotW, and Cannon's Core) tend to be very short, usually lasting no longer than two minutes.

This isn't even questioning more recent games with the boost, as accommodating alternate characters with this kind of playstyle has never been attempted before, and to be honest, really escapes me as to what could possibly be done about it. But barring that, the question still remains, how exactly do we design levels in 3D with characters like Tails or Knuckles in mind? To boot off some initial points of discussion, I think Tails' levels should play a lot more with verticality; levels that challenge you to find ways to scale them from the ground up, using intuitive use of platforms and space to make your way to the top. I really don't know what to do about Knuckles' though, aside from constraining his level design to be more like hallways (a la Death Chamber) to avoid massive sequence breaks. But what do you guys think?

[Muscles Maximum]

This is something I've wanted to make a topic about for a long time but never did because I couldn't really get my point across well enough to promote discussion. 

Still it's definitely possible to make Knuckles perfectly playable and not broken in 3D. The few non-treasure hunting examples we have are Sonic Heroes, Black Knight, and 06. In Sonic Heroes Knuckles couldn't climb walls, and his glide was hyper nerfed and turned into more of a slow descent. There are ways the glide can work in a slightly more traditional way, but overall, Knuckles' signature abilities were nerfed really heavily so this is something we'll want to avoid.

Next we have Black Knight. Sonic and the Black Knight only has super linear levels where you are literally in a hallway with invisible walls for the entirety of the game. Already this weakens Knuckles' gliding and climbing. This we'll want to avoid too since I'm sure most people don't want a game that restricts your movement this much. Aside from that, you could only glide and climb for a limited amount of time. Still, since the game is so linear these are rarely problems.

Lastly we have Sonic 06. Surprisingly, this is probably where Knuckles works best. Well, on paper, not mechanically. Knuckles can climb on most surfaces and has an 'infinite' glide like in the Adventure games. However, the gliding has changed in that Knuckles descends, very, very quickly. This makes it so you cannot climb up high and simply glide to the end like you can pretty much do when you put Knuckles into Sonic's levels in SA1 or 2. 06 also gave Knuckles a super cool Screwdriver Punch attack that effectively acts as his own version of a homing attack. Except he can't do it while in the air. You need to be grounded, hold the b button to charge and release to spiral into an enemy.

Should Knuckles ever return fully playable in 3D (Please come back) I would like him to function more or less like a mix of Sonic 06 and SA2, except of course without 06's broken mechanics. For grounded abilities, I'd keep his normal B attack punches, because why not. The screwdriver punch is a must have for 3D. It's satisfying to do and makes defeating enemies easier. Though I'd probably lessen the charge time on it a bit to flow a little more smoothly. I like the charging because it's more 'stop and go', it feels powerful, and I feel makes a nice contrast to Sonic's more quick and versatile Homing Attack, so it suits Knuckles really well. Of course, I'd also keep the rolling that was present for him in SA1. I seriously loved that mechanic. For aerial abilities I'd make his glide more like 06, you descend faster and at a greater trajectory the longer you remain airborne. I like this because it makes Knuckles feel 'weighty' and it just kind of feels right for him. Climbing could just be a little bit more restrictive with what surfaces you can go up. I'd imagine the levels could be designed in a way where lots of climbable walls have lips coming outward so you can't pull yourself up to get in an unintended area or something. It'd take some work but it could be done. I'd also keep the Drill Claw to act as his 'Stomp' technique to come straight down quickly. Digging into the ground and quickly coming back up like SA2 would be cool too, but not totally necessary because--

-Something I'd love to see for playable Knuckles is digging that acts similarly to the drill wisp. I think I would limit it to certain 'digging sections' and if you're in 3D it would funtion more or less like the drill does in Tropical Coast in Lost World, and if you're in a 2D section obviously it'd act just like the drill wisp does in 2D. This suits Knuckles beautifully and when Colors first came out I thought 'Man, Knuckles needs this when he becomes playable again'. 

[Diogenes]

32 minutes ago, Writer's Blah said:

... the question still remains, how exactly do we design levels in 3D with characters like Tails or Knuckles in mind?

That's a hard question to answer without knowing what kind of baseline gameplay we're working with. In general, though, I think it's going to take a significant amount of nerfing or reworking of their abilities, not simply trying to design levels around them.

The thing about Sonic games is that their heart and soul is your relationship with the ground. Tails and Knuckles' abilities are both all about being off it. 3&K managed to balance them well enough both because of the more constrained nature of 2D sidescroller level design and because of some limiting factors built into the moves that the 3D games have tended to overlook (which, in their defense, did make the characters feel more fluid when using their abilities than they did in 3&K). The moves are going to need to be tweaked to be less abusable and to keep the characters more grounded so the player is still engaging in the core "Sonic" gameplay while still remaining useful and feeling appropriate to the character.

To throw some ideas out there, swap out Tails' flight as his basic midair move in favor of a slow descent or maybe even something like Yoshi's flutter jump for a little extra height. Put it in a similar class as Mario's raccoon tail or cape slow descent, something to extend your jump distance and give you more time to position yourself before you land back on your feet. For his actual flight, I wonder about it being something you have to charge up beforehand (while on the move, I'm thinking). Give it a nice fast launch, like a super jump that he can parachute down from (and drop out into a spin with a crouch/roll button), but limit his horizontal speed to keep it as more of a vertical movement tool than a fly-past-everything tool.

For Knuckles, limiting what/where he can climb is important. Climbable surfaces should still be common (definitely more than they were in RoL), but it needs to lean away from "climb everything" so that doesn't become the default solution. Maybe keep his climbing speed relatively low to discourage relying on it excessively, but give him a way to heave himself onto his feet to run along the wall (though I'm not sure how or to what extent you'd be able to aim where you'd start running). Gliding probably needs some amount of decay to it so he drops faster over time instead of maintaining the same falling speed. I've thought about some kind of "swooping" mechanic, like a lesser version of SM64's Wing Cap, where you can dive to trade height for speed and pull back to trade speed for height, but I'm not sure if there's enough space within a normal jump to make that work or if it might lead to the same problem of being too air-focused.

I also think it might take the sting out of nerfing these moves if they were to get some fitting ground-based moves unique to them, though I don't have anything in particular in mind.

[Writer's Blah]

27 minutes ago, Diogenes said:

For Knuckles, limiting what/where he can climb is important. Climbable surfaces should still be common (definitely more than they were in RoL), but it needs to lean away from "climb everything" so that doesn't become the default solution. Maybe keep his climbing speed relatively low to discourage relying on it excessively, but give him a way to heave himself onto his feet to run along the wall (though I'm not sure how or to what extent you'd be able to aim where you'd start running).

That actually sounds like an interesting mechanic. It does seem like Knuckles in particular needs some significant nerfing to make him more workable in 3D. Alternatively, instead of outright limiting which surfaces Knuckles can climb, what if he had an endurance meter instead? What if certain wall materials like metal or concrete caused Knuckles to tire out more quickly and then starting sliding down the wall when the meter ran out, a la the cat suit from Mario 3D World?

[expansivelovestories]

It is possible that Tails and Knuckles would be able to return as fully playable in 3d with their original movement styles if the zones become more similar to open world types of environments.  
Even the adventure fields from the first Adventure game are closer to that than not; This would not require strictly an "open world Sonic game", but, just as the topic-starting writing explains; more vast zones with heights and subterranean, aquatic, and other types of subenvironments that some characters could navigate but not others, would help this to happen!  
The engines just haven't been able to support this yet; Sonic has been able to really shine in the meantime while still at least including many other characters within the games.  

It really does seem like it is Tails and Knuckles that are already prepared to be part of larger zones. It is Sonic's gameplay that will have to change a bit, which, Lost World's added navigation abilities show it is already possible!  Similarly the well loved animation sequences accompanying Sonic CD show that Sonic would someday make sense in huge 3d zones.   
The same experiences described in early and ongoing presentation of Legend of Zelda; Breath of the Wild, being able to see very faraway locations and reach them essentially on foot, makes me think of Diogenes' mentioning the relationship between Sonic and the ground!  

Overall I would be equally surprised, both if they did, and if they did not, try to do something remotely similar to this at some point in the future!  My guess that Nintendo would allow Sonic Team to try out, and adapt, their open world engine may be very incorrect or it may not, given the positive interactions between them now! 

[Blacklightning]

Far be it from me to use SRB2 as a case in point for anything any other time, but I feel like they had a pretty good knack for it as far as balancing characters and level designing around them was concerned. Tails was good vertically, but keeping speed while flying was even harder than keeping it on the ground - you could manage some crazy distance from a spindashing start, but turning and strafing was too loose to handle anything delicate with it beyond a relatively straight line. Knux was kind of a jack of all trades - his gliding was about as sensitive as you could wrangle your mouse, and he can theoretically get anywhere combined with climbing, but he's slower than Tails in the air and slower than Sonic on the ground. Sonic was more rooted to the ground and generally harder to play for it, but he wrecked shit there, to the point that he could close gaps on the other two pretty easily when played well, even accounting for the corners they could cut that he couldn't.

It's easy to point at Sonic's level design in SA1/2 and claim anyone else would be overpowered there, because that's just the thing - those levels were designed for him and him alone. The reason sections of level are surrounded by ocean or bottomless pits is because they're made in mind for a character that can't fly, and in many cases this makes a more organic barrier than simply putting a giant wall in the way of places the level designers don't want you to be yet. Hell, many of the shortcuts in Tails's sections read like oversights and exploits rather than intentional design, because I don't feel like anyone in their right mind making a game in a competitive environment would ever put the exit of a level within flying distance of the starting point on purpose.

The main reason that seems to obstruct discussion of this is the same reason that seems to obstruct discussion of 3D genesis physics - because Sega practically gave up on it before they even went full 3D, so we don't have any real examples contextualized by the Sonic franchise itself yet (unless you count 06, in which case, fuck you). I don't feel like this is because they can't work so much as it is that Sega/ST are too lazy to make it work, because it's not like dissonant characters in a platformer are particularly unheard of, 3D or no. All this really is, is a cue to be careful with the balance of character abilities on the drawing board, and to actually have a decent QA department so they can catch ridiculous skips before the speedrunning community does.

[Salamander]

This is a really interesting topic.  It seems to me that part of the tension between multiple characters in 2D versus 3D comes with the difference between the 2D ideal of every character playing like Sonic, and the subsequent 3D ideal of every character having a more individual playstyle.  The latter principle makes it much harder to design and implement multiple playable characters unless they're some kind of variation on Sonic, lacking in noticeable extra abilities.  I do think that, to a certain extent, this has to do with the idea of economy.  The more you develop additional characters into something beyond a simple bonus, then the more money and time you're putting into it, but is that expenditure being rewarded by actual play time?  I'm thinking of such statistics as those indicating that most people who begin a game never actually complete it.  If only a small section of players are actually playing your bonus material, is it worth inserting?  (I understand that this is the reason why the Battle Frontiers in Pokemon haven't appeared recently, for instance.)  So I think this is part of the reason why extra characters started to become less of a bonus and were integrated into the main experience; if you wanted to complete the game, you didn't just have to play as Sonic, but to play as Tails and Knuckles in levels tailored towards them.  However, it's at that point you run into difficulties with people who didn't actually want to play as Tails and Knuckles...

You make a solid point about how Tails's and Knuckles's mechanics have a greater risk of trivialising 3D level design than they do 2D; however, I do think it's worth highlighting that, for me at least, playing as Tails and Knuckles in S3&K always did feel a little bit broken.  I think that part of the trade-off of exploiting those "broken" abilities was that it was kind of boring to do so; you could fly or glide over whole levels, but it wasn't a particularly interesting experience - even climbing for very far wasn't interesting.  Knuckles was at his best in areas of the level set aside for him specifically, which seems to anticipate and favour the 3D interpretation of having separate characters with wholly distinct level objectives.

I'm impressed by the arguments above for how to rein in the extraordinary abilities of Tails and Knuckles - with a kind of height cap on flight,   It's noticeable that Sonic Boom, and Shattered Crystal and Fire & Ice especially, do seem to take this into consideration, even if people often didn't like how it was realised; specific climbable walls for Knuckles, Tails's flight limited to updrafts.  (They do have Drill-Wisp-style areas for Knuckles, though, I believe.)  Nonetheless, I do wonder if we're neglecting the possibility of designing 3D space differently.  The 3D games have tended to favour what's sometimes called "highway in the sky" level design, where you're restricted to a long corridor in the centre of an abyss.  This provides spectacle, certainly, but wouldn't corridors with walls rather than drops make abilities like Tails' and Knuckles's less effective through containment?  This is essentially an argument for a greater emphasis on platforming in 3D rather than pure racing and obstacle-dodging, though the simple fact is that platforming and camera control are much more problematic in 3D space overall.  Perhaps this is why some argue that Sonic has never really worked in 3D; platforming in 3D is one thing, but fast platforming starts to become suicidal - although this is another issue which could be alleviated by not suspending levels above a gigantic bottomless pit.  Time was that failing a jump took you to a different path through the level, but these days failing a jump just kills you instantly...  I suppose what I'm suggesting is that 3D Sonic gameplay has a lot of problems, but perhaps they aren't quite so inherent to the concept of 3D Sonic as some would suggest.  The use of Galaxy-esque gravity gimmicks to allow spectacle to exist alongside a situation where it is difficult to "fall off" would, I think, go a long way; I think Lost World made some gestures in this direction with its cylindrical levels, though the 3DS version at least didn't seem too interested in pursuing the idea.

[Tara]

I'm not really convinced that the examples you give justify the difficulties of making Tails and Knuckles playable in a 3D space.  You put Knuckles in Emerald Coast, a level tailored specifically to Sonic's abilities, and of course he's going to be a little broken.  The linearity doesn't have to be a problem in this regard.

Either make Tails' flight less of a game-breaker by changing how quickly he can move while flying and what he can access while doing it, or alternatively build the stages around said flight.  Tails' run in Speed Highway is a good example of this.  There's still a lot you can skip over, but mechanics were generally made with Tails' flying in mind.  The majority of the level takes place on the high rooftops of Station Square, and there are boost rings in place specifically for you to fly into.  There's not much room to dawdle, because it's not competing with Sonic's playstyle as much as amending it.  By contrast, Windy Valley was pretty much designed in the same manner as a Sonic level.  It takes place high above the clouds, sure, but movement is still directed not by floating platforms, but a predefined, narrow path.  It's not to say that floating platforms are the only way you can do this, but floating platforms are a much better alternative to take advantage of Tails' flight because it forces you to make decisions on how far you think you'll be able to fly before getting tired and plummeting to your death.

Knuckles also doesn't have to be so hard.  Give Knuckles enemies that correspond to his abilities of gliding and climbing.  Put obstacles on the wall that aren't so easy to avoid but can offer a much faster route if you do manage to avoid them.  Put air hazards in the way when gliding is required or expected.  As Diogenes mentioned, limit what walls Knuckles can climb.

The problem with many of the stages in SA1 is that unlike their Genesis predecessors, they're mostly meant for a single, pre-determined path.  So Knuckles and Tails having increased control over the Z-Axis is only a problem because the level design makes it that way.  In fact, it's arguably a problem with Sonic's gameplay, too, since it feels like speed is less integral and more of a given.

[Sparky]

The core abilities of these three characters, Team 'Heroes', are Speed, Flight, and Power. Sonic was designed to run fast, Tails was designed to fly through the air, and Knuckles was designed to break through walls. In addition, Flight and Power are essentially off-shoots of Speed: Flight is Speed+Levitation, and Power is Speed+Strength. The abilities of Tails and Knuckles, at their core, are pretty much tailored specifically to allow them to traverse through the zones in such a way that they can potentially go from A to B much faster than Sonic can, despite Sonic being the fastest thing alive. Tails was created to be Sonic's sidekick and best friend, with the purpose of following alongside Sonic during gameplay. So, Tails was designed to be able to fly through the air so that he can keep up with Sonic's speed, and his ability allows him to skip over most obstacles and use the sky as a shortcut to quickly reach Sonic's location when Sonic leaves Tails behind. In Sonic 2's story, it's this ability that allowed Sonic and Tails to become friends, and in Sonic Adventure, it's the basis behind Tails' gameplay of racing Sonic to the goal. For Knuckles, he was created ultimately to be Sonic's Rival, and pretty much everything about him is specifically designed to counter Sonic in some way. Such as: Knuckles has a purpose and responsibility keeping him locked in one place, Sonic has the freedom to travel the world in search of adventure. Knuckles was designed to be powerful and tough, being able to break through barriers with ease, to counter Sonic's speed. In Sonic 3K, a relatively unnoticed gameplay mechanic that Knuckles has is his ability to easily smash straight through any unbreakable wall he runs into, where Sonic&Tails would be stuck pushing against the wall, or needing to charge a spindash. Of course, everyone seems to pay more attention to gliding and climbing for whatever reason, and they also appear to function as a counter to Sonic's speed, because is easily apparent in a 3D space is that Gliding through the air and Climbing up walls are abilities that lend more towards exploring a vast open space and hunting down hidden treasures, instead of blasting through with sonic speed to get from A to B as fast as possible.

A possible reason for most 3D Sonic games being designed in a linear fashion is because of this 'get to A and B as fast as possible' approach to reinforce the idea that Sonic is about Speed, so being able to explore would slow you down and works against your speed and racing against time. However, what we've seen before is that 'exploration' in a Sonic game is multiple pathways and being able to take different routes to get from A to B not only quickly but in unique ways and possible finding which route is actually the quickest and best to take. This is something that is actually exemplified in Knuckles' core abilities whenever Gliding and Climbing aren't as present (such as Heroes or Riders), where his Power can be used to break through barriers in order to open up new hidden blocked off routes that function as a shortcut. 

Anyways, in my opinion, Zones just need to be designed as vast open spaces between A and B, allowing Sonic, Tails, and Knuckles to travel through using their unique abilities without anyone needing to be nerfed or restricted. 

[MetalSkulkBane]

I have very lazy solution. Make game with 3D levels and 2D levels

in 2D you play as Sonic, Tails, Knuckles or Amy (maybe also Cream), just like in Advance series.

In 3D you play as Sonic, Shadow, Silver and Blaze (maybe Metal Sonic). Why?  Because all of them can be molded into Sonic with a little gimmick. (Really useful if we still use Boost gameplay)

Some characters will be tricky (Silver), but in the end pretty much all major Sonic characters would beplayable (sorry Chaotix). Of course making all of them part of the story would be hard, most likely part of them would be just unlockables (which would satisfy fans of characters and those who don't want for Sonic to share time with his friends).

[AngelSlayerN64]

36 minutes ago, MetalSkulkBane said:

Some characters will be tricky (Silver)

Silver wouldn't be tricky. Just make him fast like he was in the Sonic Rivals series and Generations 3DS, or better yet...

His gameplay style in The S Factor: Sonia and Silver could be another plausible option.

You could also do what Joshscorcher suggested and turn Silver's gameplay style into Katamari Damacy with Telekinesis and try to maintain the "combo" by using precise PSI timing.

It's really not that hard to incorporate. You just need some imagination and effort to make it real.