No offense to any Wario fans, but I don't really want to see any Wario characters in the Mario Kart series. I just don't see them being good additions other than Warioware players, and it would be like MSM roster where there are just some random characters that aren't in the normal Mario games and it'll just be weird.
I can understand Yoshi and DK characters since they are more tied to Mario in many ways (especially the Yoshi series), but Wario has really been a separate thing from the Mario series. I mean, I don't mind Dixie coming in as well, but I don't want the Mario Kart series to slowly be taken over by the Kong family, and being less of a Mario Kart game. I mean, 3 Kongs appeared in Mario Kart Wii, which is quite a lot for a non-DK game.
Now, I will say that even though I do find the species problem to be a...well, problem (excuse my lack of better word choice), I honestly don't see it as that much of a problem. I mean, generic characters I see being less of a problem than adding some rather minor characters like Nabbit, or when they added Honey Queen. Like, I think that having a generic Koopa Troopa or generic Lakitu is better in terms of favorably to that character than specific minor characters (more people would like that character if it was more generic rather than being like HQ where only a few people like that character because of the specific character). Just because there isn't a stand-out character out of the species, I honestly don't mind. I mean, heck, I would love to play as Blooper as a Mario Kart driver, and I don't think that it should be shunned upon if that character was added instead of perhaps a somewhat minor character. I mean, sure, it lacks uniqueness a bit and does take a space that a specific racer could have had, but this is kind of like a first world problem in terms of Mario Kart issues. I only see it becoming more of a problem when they start making more generics than they do unique characters.
I'm sure that many of you disagree with my opinion, but that's how I feel.