I think they're going to stay relatively close to the established mushroom kingdom, instead of branching out, as it would only make people ask "where's that character from?" or make them wonder why a well-known Mario character was dropped in favor of some weird character that appeared in one spin-off game.
I mean, some people are already saying that Diddy Kong is Aiai, and Aiai isn't even a Nintendo character!
I think they're going to put Toad in, as he's a favorite among a lot of players and isn't on the main roster. After that, though, I have no idea. I mean, I'd imagine they'd have to have a brain enough to pilot a kart, so while the Troopas make a little sense (and indeed there's the standard and para variety, although I'm not sure what the real difference is besides the wings that may not even do anything), I don't think they'd have a goomba (cos they seem really dumb). I'm thinking they might have another character from SMS, perhaps Petey Pirahna? Or a throwback to smb2 with Wart? I don't think they'd add another Koopa Kid, because they have Baby Bowser included.
Maybe they'll have Toad and a Shy-Guy, since they seem keen to those guys. Also, knowing how Nintendo's been doing a crapload of unlockables in their "party" games, I'd imagine they'll have a crapload of unlockable battle arenas, too, a la ssb:m. I could see them including the 4 N64 tracks, perhaps with slight modifications, as a "pack," and then provide, say, a battle course for each pair of characters, too, for 8 battle tracks, and maybe a few unlockable ones.
What I wonder is that if unlockable characters will still fit with the current crop of karts, or if they'll also have a few unlockable karts as well.