I really hope Nintendo doesn't add anymore relative characters, I dont wanna see the entire cast of Mario crammed in there, it just takes any chance of another main character to step up.
For example, if I have Mario, and Luigi (Toad would be ok) im fine, I don't want to see 346262362362356 variations ranging from paper, to fire, to tanooki suit. Nor, do I want to see everyone crammed in there to all 7 of the Koopa children, to Birdo, the clouds with smiley faces, those damn creepy trees from Super Mario World, and so on. Let the main characters (maybe some secondary) have a chance to be in first.
Personally, i'd like to see a push for some more 3rd party characters. Who wouldn't want to see Sora bringing the keyblade down on Kirby, or Fox? (Never happen, but a fanboy can dream)