Ditto.
I'd imagine we'll get the standard Goomba and Koopa partners for the first two areas in addition to the Chain Chomp and some brand new character (Though similar to other characters, Watt and Flurrie are pretty unique species). That leaves about 3-5 spots for other partners. Of the species they have not chosen from yet:
-Shy Guy
-Blooper
-Piranha Plant
-Swoopers
-Clefts
-Clubbas
-Rough Puffs
-Wigglers
-Hammer Bros./ Koopatrols
-Toad
-Crazy Dayzees
-Fuzzies
-Podobos/ Embers
-Bandit
-Pianta (Who, for some reason, I'd imagine might be pretty cool even if it is a Pianta)
I probably missed a bunch, but those are what I came up with off the top of my head. I'd say Shy Guy and Toad are the two most likely to appear (Toad in particular, considering we've seen fighter Toads in both of the RPG Paper Marios), but who knows.