As I said on SNN (A Sonic wiki I am an admin at), I believe if they made a Sonic game with at least some MMOG features it would made that games shelf life last more longer than it's market.
My idea would be for the next Sonic game to have a "side game" called Avatar Battle Mode. It be like Battle Mode from Shadow's game but with Avatars and wispons. As long as it has more than three stages and more types of competitive modes then Avatar Battle Mode would carry the whole Sonic game for years.
Don't believe me? When Fortnite was release, it was released as a game with it's own story mode while the battle royale was a free seperate mode. In yet the free battle royale of the game has pretty much taken the entire identity of Fortnite and been keeping to it's audience interest for years!
If a Sonic game could copy what makes this MMO games so popular it too could make a Sonic game that's marketed for 2-3 years last for yeard on end.
Would love if the Avatar Battle Mode was expanded to be like PSO2 in layout.