This one is admittedly the most probable on this list due to R.O.B. having actually already appeared in Mario Kart DS. In fact, the robotic operating buddy even has the distinction of being the first non-Mario character to be playable in a Mario Kart game. Take that, Link.
Despite Mario Kart DS being R.O.B.'s first and only foray into the racing world, his renewal on the Super Smash Bros. for Nintendo 3DS/Wii U roster proves that he has not been forgotten. Perhaps someday we will get to see those innocent robo-eyes behind the wheel of a kart again.