Hrm good luck with this. Never been 2 Mario Karts for one system yet. And a Super Smash Bros and Zelda have yet to come out, so nintedo will probably focus on that. I certainly hope a SSB comes out. We know a Zleda is, man they told is way to early, gonna make us wait and beg :'(
Anyways if they do yeah new idea would be nice.
Examples
Smoke bomb item, sends smoke behind you, sorta like the ink blot but you can see at all, however, you ride through it and its over.
Make blueshells avoidable in some way, like maybe a mushroom can speed away at the last second. Cause the AI is really cheap with them.
Bring back coins from SNES for a speed boost, and the koopa troopa, though dry bones sorta replaced him.
Custom online race scheme. For instance, you get to choose which items can be used, if powerboost is on/off
A rearview mirror option.
MAKE BATTLING ONLINE(sorry this needs caps)
Read above again heck read it a third time maybe even a fourth.