CTGP-7 is a great modded version of Mario Kart 7 that allow for a virtually infinite, user-generated roster.
I've made a few mods for it, including 1 kart and 2 characters:
A Bad Piggies vehicle as a kart:
https://gamebanana.com/mods/660478
Luigi from "Luigi Clash Mario" as a driver:
https://gamebanana.com/mods/658801
Purple Guy from "Five Nights At Freddy's" as a driver:
https://gamebanana.com/mods/688350
