That Mario Sunshine total conversion should have been its own thing.

Take Rollin' Rascal; it's an indie game that plays almost exactly like Sonic and you even have a ball form. But everything is legally distinct, and it's a super fun game. There's a mechanic for "taking over" different robot enemies so you can use their powers on others or the environment to unlock secrets and shortcuts. The company that's making it previously worked on (I think) a couple Sonic games, and I believe they're working on a port of Mother 3 as well.

Edit: there's a demo that they keep updating with cool new features, and there's a multiplayer mode with races and different tracks. You can also customize your "rascal" with different colors and outfits. All free and incredibly fun.