As far as creating a kart goes, obviously creating a whole new model for a kart is sort of dumb for this type of game, but I hope you can at least have some options for customizing your kart. Like selecting your own color scheme, maybe have the ability to draw a logo or emblem with the stylus (like F-Zero GX but easier), and tune the kart and be able to customize how much acceleration, top speed, etc. you want.
It could be cool to have more than just the normal kart mode too. Diddy Kong Racing had hovers and planes which weren't bad.