If you are going to call it Mario Kart you need to keep it to just Mariouniverse characters...in which there are plenty.
But I would love for the next Mario Kart game to be called Nintendo Karts, or NintenKarts or something, and have characters from all Nintendo games in there. Yeah, in a way it would be pretty stupid, but in another way it would be campy cheesy fun. And you know what...before Smash Brothers came out we never would have accepted a fighting game mixing Nintendo's characters together.