Argh! Dammit, right in the feels. What IS Sega thinking?
Playing as different characters is fun. Raising Chao is fun. Having Sonic's levels actually be uninterrupted running into the screen GOTTA GO FAST (rather than constantly having to stop to solve puzzles or switch characters within the same level or turn into a werewolf or some stupid shit) is even more fun.
And now that you mention it, a ponified version of the same concept - six interlinking stories, one for each of the main ponies, each with their own gameplay style - would be amazing.
Rainbow Dash = Sonic (gotta go fast)
Rarity = Knuckles (find gems)
Not sure about the rest. Maybe Twilight is like Gamma or Tails in SA2, blasting through everything with her laser horn?
And a Fluffy Pony chao garden. Teach them to love Fluttershy, and fear Rainbow Dash!
Dammit, now I wish I could make such a game, but alas, I can't into quality 3D game programming. And these days, apparently neither can Sega.