I actually realized awhile ago that the Mario series has the most characters I would like playable in smash they are:
Characters who have a shot:
Bowser Jr., Waluigi, Paper Mario, Kamek and Toad
Characters who have little to no chance of inclusion:
Petey Piranha and Fawful.
---
Bowser Jr. has the best shot to me. He has a unique weapon and has been in every main Mario game since his first appearance.
Waluigi has a pretty good chance at this point as well after being an AT and most fans have warmed up to him (myself included)
Crotch chops and Brawl in the Family helped with that. When Brawl came out I despised Waluigi actually, but now he's too weird no to like a little.
Paper Mario could go either way: on one hand he has pretty big potential and a pretty big fanbase. On the other hand they might avoid his inclusion because he is just another Mario. It's hard to convince some Smash fans that Toon Link is important after pretty much being a sub-series of Zelda let alone a flat version of Mario.
Kamek Though for him to even be considered he would have to be dubbed a Yoshi rep, but with that in mind he is the best choice. The only other options are a generic Shy Guy who is...well,...a generic Shy Guy, and the Baby Mario duo. The duo is a bad choice to me because even the majority of Mario fans (that I've come across anyway) dislike them, and accuse them of eating up space in spin-offs. Kamek, meanwhile, has a major role in nearly every Yoshi game, unique moveset potintal, and no one despises him. (that I know of)
Toad...poor Toad. At this point I don't think he will ever be playable. If he dosn't make the leap from attack to playable in SSB4 I think it is officially time to count him out. I honestly don't care either way anymore; his role as a move for Peach fits him remarkably well. There is something very appealing about having every SMB2 character playable though.
---
Petey: My favorite Mario character by far. He's big awkward speedo wearing flower with attacks built around spewing goop. After his apperance in SSE I'm not counting him out completely yet. Ridley is more then likely going to shrink from his Brawl size to fit into the roster, and there is no reason Petey can't follow suit. I'm not holding my breath though.
Fawful: It's Fawful. Do I really need to explain why I like Fawful? He's quirky... he's been an antagonist twice now...he has a pretty large fanbase...he has movset potintal...and he has fury. I'll be lucky if he even gets a trophy.
---
But like everyone else has said I doubt we are getting more then two new Mario characters. I would honestly be shocked if we get three or more.