Super Sonic Galaxy 2?

[Vipershark]

the only problem is that mario and sonic have totally different gametypes;
sure, they're both platformers and their main form of attack is jumping on things, but that's about where the similarities end.

Sonic is all about speed, hills, loops, and action.
His levels are mostly dynamic sloping levels that aren't just straightforward. (I'm talking classic sonic here, though "speed boost" sonic still fits this.)
Not to mention the special stages and chaos emeralds, as well as super sonic.

As for mario, the levels are much more straightforward, and not as hill-y or whatever, as mario isn't totally about speed.
It's generally a much slower paced game, and has more puzzle solving and platform jumping elements.
Not to mention that the powerups wouldn't really work for sonic. I suppose it could be argues that the fire flower could give him the fire shield and the star would be invincibility, but that's pretty much it. Mushrooms and mario's other powerups don't make sense for sonic, and vice versa.