Bikes. I know we aren't getting it, but I sure as hell am disappointed they booted bikes. =/
This probably won't be possible on the 3DS next gen Mario Kart, but custom course building. Since the 3DS won't have as much storage room and the like, you could essentially have copy/paste race courses (but a LOT of them), multiple different sorts of textures and themes for the course, and all the essentials to make said course. i.e. you decide placement of boosters, obstacles, item blocks, etc. and all that jazz. The WiiU Mario Kart could definitely make huge use of this, but just implementing the idea on the 3DS, even if it's not fantastic, would be cool. Not that this'l happen either.