A long time ago I talked about the idea of another Mario 64 remake with 4 playable characters, but this time those characters were Mario, Sonic, Crash, and Master Chief. And I mentioned how while it's a neat idea on paper, there are a few problems with actually making the idea a reality.
One of those being that Sonic's high speed would make him nigh unplayable in over half of the levels in Mario 64. To help emphasize this, here are the levels Sonic could actually be playable in. Not counting small bonus courses like Peach's Secret Slide.
Bob-omb Battlefield
Jolly Roger Bay (Debatably. It would depend on how Sonic would deal with water in Mario 64. This same thing applies to all water heavy levels in the game.)
Big Boo's Haunt
Hazy Maze Cave
Dire Dire Docks
Snow Mans Land
Wet Dry World
And that's it. Only 7 out of the games 15 main levels. 4 if you believe he couldn't be used in the water levels. All other levels, including the 3 Bowser Levels, he'd be screwed.
also why is wario so smiley