Well I would like to see Boo, Dry Bones and Hammer Bro. playable again while keeping Koopa as playable and to see Wiggler and Bowser Jr. having another appearence in the game, as well as some other characters that we are seeing a little everywhere which could be fun to add around.
So, nearly everything which could be Koopa-related, like the Koopalings, especially Larry Koopa and Lemmy Koopa or simple characters like Bob-Omb, Goomba and others... it's just because I really like them and would hope to see these appearing in a Mario Party game eventually (or maybe a surprise with someone else unexpected I like that could be playable) but I'm sure that most of these might not have a chance at all, but there's still hope for the future games even if it's not the case.