I'd rather Mario Kart keeps itself on the Mario franchise, thank you very much. Crossover franchises would be out of place really.
They can pull out QUITE a few characters with just the Mario franchise (and by that I also include the subfranchises like Yoshi, Donkey Kong and Wario)
Other than that, I'd like more dynamic stages, both faster paced crazes and slow paced ones just to gawk at the scenery (which should be awesome in case the NX is how it should be)
Underwater, ariborne and anti-gravity sections are welcome, and maybe a "Double Dash" mode to have two drivers on a single kart like in the game of the same title.