Well, geez, you wouldn't even need to list the characters - you could pretty much say it has the same or similar roster as any Super Smash Bros. game.
Though "Super Smash Kart", "Super Smash Tennis", "Super Smash Party", etc. could be kind of fun, I think Nintendo would avoid further oversaturation and crossovers of their games. I could be wrong, of course...the money is there, they could easily sell a lot of any mega-crossover game - I for one would like to see an RPG in the Nintendo "universe", as such crossovers could be potentially epic if they're not silly or just for the sake of a "dream match".
A "Super Smash Kart" game should be significantly different from Mario Kart, unless it replaces all future Mario Kart games. But if it's too different yet is still kart racing, that would be awkward, so replacing the series makes more sense. And with the Wii remote's new control possibilities, Nintendo would do better by releasing another Mario Kart game using those controls and consider a "Super Smash Kart" only if they ever run out of ideas to add to a Mario Kart game.