I would rather not have Megamix be the template everyone does their hacks on. Then all hacks would be like Megamix, or even "I palette changed Megamix this is my new hack tech member pleez!!1"
What I think would be best is a custom basic empty template dissasembly engine whatever that mimics the classic sonic games for people build their stuff on. Something that isn't Sonic 1, 2, or 3K but mimics them. They wouldn't be hacks anymore, but homebrew genesis games. Also, it would be more legal, right?