I think they should have a few "guest characters" each game, but not a full blown Smash Kart type thing.
Like they could add say Toon Link, Fox McLeod, Olimar, and Captain Falcon and then have a 5th Cup that's unlockable which has a Zelda WW stage, a Star Fox stage, a Pikmin stage, and an F-Zero stage.
I would be cool with that.
4 unlockable "guest" characters per game each with a corresponding stage (makes a 5th bonus Cup) would be OK.