Maybe it could be a somewhat good idea to add a create your own track feature in the next Mario Kart. But of course you'll be able to play it offline and only with your friends on wi-fi so no one would be like "This noob kept spamming his custom track online and I kept losing aesoplopdfvatiw......"
You would be able to make roads of many kind: normal, icy, rocky, bumpy..... You could widen them or slim them down, add boost pads, trick pads, jumps, obstacles like SGB crabs or Goombas, make certain parts of the road curvy/slanted like Rainbow Road (better example would be F-Zero. Along the lines of Cosmic Terminal in GX). They could be as long as you want but obviously there should be a limit. Maybe any other features you could think of.
It should be better than the Custom Stages in SSBB. I mean it shouldn't be hard to make a decent racing track if you have enough features to do it.
What do you think?