Sonic Boost Racer - Basically it's Unleashed daylight stages in a game that is a mixture of racing and platforming, in a wacky race style, with the due alterations to support the number of racers/players, like larger stages/courses and such things. It would be a new genre of game.
Tails and Knuckles: Chaos Quest - Inspired by Ratchet and Clank, Banjo-Kazooie, Yooka-Laylee, Jak and Daxter, Mario & Luigi. So many games have two characters to play with. It could even be a smaller series, like Nintendo's Mario and Luigi. Sonic has a history with this thing too, given Knuckles Chaotix, Sonic the Hedgehog 4 Episode II, Sonic Heroes and the ability for a second player to control Tails in Sonic the Hedgehog 2. In my pitch, Knuckles uses strenght and Tails brains, you have to alternate between the two to figure out how to procceed in the levels so it's not all about speed. It's more a puzzler, having some elements from Tails old adventure games. It kinda seems what Boom tried to do (I didn't play Boom to be sure). Tails also uses gadgets ala Clank.
Lego Sonic - Self explanatory. Why doesn't it exist yet?
Sonic Force - Not to be confused with Sonic Forces. Inspired by Mario + Rabbids, it's a Sonic SRPG game in the mold of Shining Force classics.
Sonic Expansion Project - This is the idea I like most. Consists of releasing in a digital form games to flesh out Sonic's world and mythology, each featuring a different main character. These being smaller, indie-like games. They would be games like Blaze the Hedgehog, Silver the Hedgehog, Espio the Chameleon, Rouge the Bat... and would feature gameplay to match each character's skill set.
Hedgehog Freedom - A spin-off totally set in Archie universe comics. IMO it's what Sonic Boom should've been.