I dont really count spinoffs as "relevant," I mean mainstream, platforming, rpg, etc. Mario, Luigi, Peach, and Bowser are obviously the most relevant in mario games. Toad is very relevant due to his playable appearances, popularity, and how much time he has been around. Bowser Jr. is relevant because of his major roles in the last 5 years in mario gaming. Waluigi has one sole purpose: Be a partner for wario in spinoffs. Hes never had his own game, heck, even birdo appeared once in smb2. Btw, most of the spinoofs waluigi has appeared in, toad and bowser jr did as well. Thats why i think waluigi is irralavent as a mario character.
Well, you have to consider that Mario Kart Wii as well as MK 7 are indeed major titles and is one of Nintendo's biggest sellers.
I have to concur with you though, Bowser Jr. does indeed have a more prominent role and is quite relevant as well in major mainstream games as well.
As for Toad, he also has it going for him, the fact that he's one of Mario's most iconic characters and is part of "The Big 8" proves he is worthy of a slot, not to mention he us already avknowledged by Sakurai.
Paper Mario indeed is another candidate I can say is probable, he has become popular and also has an upcoming 3DS game which helps his chances by a demeanor.
As to who is most likely I'd say it's in between Bowjer Jr. and Toad possibly getting that 5th slot, of course if Sakurai wanted to expand on it we could possibly be getting both or if we were to see exclusive roster on the 3DS and Wii-U, for example Toad would be on the 3DS while Bowser Jr. could be on the Wii-U version.
As for Paper Mario himself, I'd say he should represent his own series as a single staple, he is popular enough to sepearate himself from the Mario franchise and more into the "Paper" franchise, of course he can easily be in the Mario franchise itself but I feel he would intervene with Bowser Jr. and Toad's chances, or perhaps they would intervene with his, unless we were to have universal rosters were they he could appear in both while have Toad in one and Bowser Jr. in the other.
As for Waluigi himself, I'd say he like "the Dark Horse" choice, he was an AT in Brawl and also seems demanded somewhat, I believe he could be represented within the Wario series, however Waluigi having no mainstream appearance isn't a factor for determining his inclusion, despite that he could very much be in if Sakurai shows interest in him which I have a gut feeling about, alsoo Sakurai can base his own original moveset on him like he did with ROB and Princess Peach to some extent, the point is he can be made to be playable regardless whether or not he has any moves or not, I feel he is slowly rising as a possible candidate.
