There already are custom racers in mario kart in the form of Miis.
But yeah, I wouldn't expect any new content for any (first-party) already-released games. Possibly some form of Call of Duty update, as they're already added a couple of new game modes to the Wii version.
Personally, I'd prefer that Nintendo stuck with the 'old-fashioned' idea of giving you the entire game when you buy it.







