Mario Kart 8 is pretty close to perfect. It is just so much better than Wii, I don't think the step to 9 will be as big. The only improvements I can think of now would be a better battle mode (although I don't really that much about it) and maybe the return of missions. I guess a level editor if they could make it work but I'm not sure how it would (at least compared to detailed levels Nintendo makes themselves).
Besides this, just more of everything, tracks, characters, vehicles, items..
Spending warm summer days indoors
Writing frightening verse
To a buck-toothed girl in Luxembourg
NES, SNES, N64, GC, Wii, WiiU, GB, GBC, GBA, DS, 3DS, Mega Drive, Game Gear, PS1, PS2, PSP, XBOX 360, Atari Lynx