I am a cynical asshole. Can be nice sometimes, but mostly an asshole.
Also, here are my thoughts on what I think the Sonic franchise should be about.
Basically, Sonic's world should just be a batshit insane geologically abnormal version of our world. A parallel universe on crack, essentially.
I mean in what world do you get skyscrapers high above the clouds, with loop-de-loop highways, with cars actually driving on them (in the background of Skyscraper Scamper).
Unleashed was a breathtaking game. High saturation, bright colourful worlds, with a striking art-style that takes the real world and puts it to shame. Seriously, I would love to live in Adabat; Dat Coast..........
That is what the tone should be. However, when the shit hits the fan and we move closer to Eggman's base, the atmosphere and the visuals should reflect that. Yes it should be Colourful, but it should feel much more menacing. Take Eggmanland and its oh so perfect description; Crimson Carnival. It looks hellish, but the saturation and Colour is still off the charts, with bright neon green and pink. I love the idea of Eggmans stages, being fairgrounds and theme parks from hell. Shouldn't overdo it though. Occasionally we should get the odd final stage, which is blue and cold, and isolated and lonely. Having both the Carnival from hell and the Cold, isolated base, would be a brilliantly subtle and clever way of showing Eggmans 2 principle characteristics. The Colourful yet dangerous Carnival from hell would represent his childhish, megalomaniac side. Whilst the Cold, and isolated space-station or ground base would represent his lonelyness (he basically has no friends outside of his Robots).
As for the plot. I think that it should be simple and lighthearted at the surface. The simplicity and lightheartedness would work perfectly with kids. Certain scenes would obviously show off how badass Sonic and his crew are. Characters interact with each other and NPC's. This dialogue should be used to develop the characters, NOT the plot. The plot should be developed by the characters actions and the sensory elements (i.e what you can see and hear happening in the Cutscenes). I hate it when characters constantly blabber out needless exposition; they're characters, not narrators. This links in to another point about subtlety. Sonic and co do not need to talk to portray the beauty or horror of a particular event. We can see what is going on, we don't need to be told. The characters can add to this by gasping, whistling or showing facial expressions. Just because Sonic and co can talk, doesn't mean they need to talk all the damn time.
Finally, I think it would be pretty neat for the stages themselves to tell their own unique little story. The characters don't talk about it, in a normal runthrough. However, accessing secret areas that are pretty well hidden, may occasionally (I emphasise occasionally, because it shouldn't happen all the time) trigger a voice clip talking about what can be seen in the secret room. For example you could come across a statue of a 'deity' in a secret room of a Ruin Zone. Or you may come across a Mural a la Hidden Palace Zone. This little backstory told by the environments, which isn't a part of the main overarching plotline, can serve as a means fleshing out Sonic's world and his world's mythology or History. In fact it doesn't have to be about the history, but about his world generally. Say if you come across an old Laboratory, it could (by having a secret collectible items a la Black Knight) show you about how Science works in Sonic's universe.
The good thing about doing things this way, is that it isn't being shoved in your face as exposition, its there, if you're looking around for it. If you stumble across it once, getting that little easter-egg of info about Sonic's world, may promt you to go through other stages to see if you can find any more. Its just a more subtle way of adding replay value. It would also send certain fans nuts.
Anyway that should cover everything I think should be in a Sonic game outside of gameplay itself.