Like a lot of people have already said I'd be happy with more DLC for 8. I wouldn't go down the route of bringing back all the tracks from previous games though, Mario Kart Wii in particular had a lot of bad tracks that deserve to stay in the past.
-Better single player content would be nice. They did some basic challenge mode stuff in one of the DS games but never expanded on it.
-Team Race Mode. Select three characters before the race, one for each lap so that when you cross the line it changes out your driver. Offer bonus points for the fastest individual character lap but weight it according to how many people picked them. So being the fastest of three Luigis is worth less than being the fastest of 11 Marios. Uncommon characters get an overall speedboost though. I'm explaining this poorly.
-Cappy as a weapon. Swap position with whoever you hit. Can be thrown backwards as well.
- Elimination Races. Take the coin strips from the F-Zero tracks and layer the on to existing tracks. Each contestant starts the race with three balloons. 10 coins=One balloon. Offer bonus points for eliminating your opponents over the course of a race.