This is a weird thread.
It’s like when someone posts a movie on youtube and then writes in the video desc: “I do not OWN copyright. No infringment intended. Film belongs to Miramax!” and they think that covers them legally.
Just don’t use other peoples work without permission. The fact that fan-games exist and not every one of them is legally targeted doesn’t mean it’s some legal grey area and you can get away with it if you’re smart or you make a good game. It doesn’t matter if it’s the best Sonic game ever.
You should just create a game with the mechanics that you want, in this case I guess similar to 3D Sonic games, but use all your own artwork and design. Save you so much trouble in the long run. Plus you’ll also save yourself the hardship of having people comparing your fan-made game to genuine A+ titles made by a company like Sega. You could use the Sonic assets as placeholder art I guess until you had your own stuff made.
Someone up-thread mentioned “doom clones”. I remember that. When did we stop calling them doom clones and start calling them FPSs?