This is already looking to be one of the best rosters so far in my opinion. All character choices thus far are perfect. A huge improvement on Mario Party 9's lacklustre atrocity. It also looks as if Spike will be playable, which is okay.
Characters I'd like to see now are:
-Boo: Them noises.
-Shy Guy: Brings humour.
-Dry Bones: So much better than Koopa. :|
-Bowser Jr.: It would be great seeing Bowser beat up his son. =p
-Goomba: If Koopa can be playable, why not Goomba? He's already used a baseball bat.
-Nabbit: When Nabbit is playable, Alfonzo takes his role?
-Diddy Kong: Don't particularly like him but others do and he would undeniably be a good choice.
-Link: Now that Rosalina's in the roster, the only characters that can be added for the most part are generic enemies. He made MK8, why not Mario Party? He's popular enough.
-Coloured characters: I don't want to see green Yoshi's all the time when I can have a variety. That goes for Shy Guy, and possibly Toad too.
I'd like to see a large roster in general, but this isn't particularly important especially looking at how perfect the roster already is now. Unless of course they finally have online in which case large rosters are always best. I do have a feeling we've seen most of the roster already, though.
What I don't want to see:
-Kamek: so boring, like Koopa but isn't nearly as iconic, especially when the roster is small and/or it replaces the best Koopa Dry Bones.
-Both Koopa and Paratroopa in the same roster. I mean, really?
-Birdo: but it wouldn't be a disaster if it returned.
-All 7 Koopalings. Please no.
-Guest characters from DK that are not Diddy Kong.