a Knuckles generations?
I instantly think of the adventure stages personally, speed highway and pumpkin hill, and if the Time Eater was involved in the story again (or Tikal via flashback stages like in Adventure) it would be cool to see Knuckles experience a non-crumbled Sky Sanctuary and maybe even a stage based off the Echidna area in Adventure
Knuckles could also get a flame core stage, since he also had a unique section in that stage, especially with that
really cool cavern music (YT link)