I absolutely adore Sonic Classic Heroes.
I hold it in higher regard than both of its source material (Sonics 1 and 2) and come back to it time after time to just try more of its new features and explore more ways to play the original games (you can find it here: https://www.romhacking.net/hacks/1038/
After playing Sonic Classic Heroes, I decided to give Sonic 3 and Knuckles a try for the first time last year, and I loved it too! However, there were several levels where I thought the ability to play as Tails or Knuckles spontaneously would be amazing (I'm looking at you Marble Garden), so I got to thinking.......
How feasible would a Sonic Classic Heroes styled hack be for the Sonic 3 and Knuckles complete experience? I'm really not even asking for more than the ability to switch between characters, with their respective capabilities intact (though I wouldn't object to the addition of the Chaotix, as well). The addition of a new ending after you beat the game with both Sonic/Tails and Knuckles would be cool too.