Really? Tough decision. I`d probably want Adventure 3 more if they changed it a lot.
Sonic - Normal Sonic levels.
Shadow - Sonic levels with cosmetic differences. (More enemies, different way to cross gaps, etc.)
Tails - Flying levels from DX, except no racing element.
Knuckles - The first radical change one, battle-style levels. Basically, a Sonic-style level for awhile, then fight a lot of enemies, run for a while, etc.
Amy - Uh, not sure.
Rouge - Stealth levels (She would be the odd one compared to the others)
Don`t know about the other characters, if any, (6 seems to be the magic number for Adventure.)
Oh, and Heroes style graphics and bosses, obviously.