Besides that, your not leaving any options for players, and throwing to much strategy into something where it isn't even needed. Combing attacks would be cool, but it should be left up to the players on who teams up, and what it is that they do, not to mention that you don't even seem to be regarding the prospects of multiplayer, cause it seems you want sonic and mario to be as one character as if this was sonic heroes. I'm not saying your idea doesn't work or that it sucks, but your letting you own ideals of what you think the characters need to be like get in the way of variety and creativity.
I mean, lets forget the ideas here for a second, what are your plans for multiplayer and other characters? Can player 2 join in and be able to pick who they want? What if they wanna team up as Wario and Knuckles or Yoshi and Shadow for instance, what does that do with your plans of having to balance mario and sonic when out of the picture, and how much more fun do you think it'll be if 4 players are in one room, but have to wait because the game is on 2 players, if you even allow that? Come man, give your players something, this is a sonic and mario adventure, 2 big name characters who have had great games in the past, coming together.