TIME FOR ME TO WEIGH IN ON MY THEORIES ON NEWCOMERS!!
Little Mac - Because the leak, and he is a fan favourite Nintendo character.
Pac-Man - Because leak, and, though I would want Klonoa, Pac-Man is being rebooted by Namco; it would make sense to reintroduce him through an appearance in the most popular party brawler ever.
Mii - Because the leak and casuals would love that.
Lucina/Marth - She is already an incarnation of Marth, and she did appear in the most recent FE game.
Chrom - Look, Roy appeared in Melee to cash in on his game; Ike appeared in Brawl to cash in on his game; HISTORY REPEATS ITSELF.
Mewtwo Awakened Forme - Kill two birds with one stone. Hardcore fans clamored for his reappearance in the fourth game, and it would be a smart way of (somewhat) satisfying the fans with an incarnation of him will being relevant to the new Pokemon movie. Forgive me, Quicky.
Waluigi - You want a better character instead? You wish for your favourite in place of this dope? TOO BAD, WALUIGI TIME.
Ridley - We desperately need another representative of the Metroid universe. I would choose Rundas but Nintendo likely completely forgot him by now. And no, he's NOT too big; Olimar was the size of a nickel in his games.
Dixie Kong - Considering she is making a comeback in the new Retro DKC game and that she was part of THE FORBIDDEN 7 in Brawl, the possibility of her appearance is good.
Palutena - Another rep from Kid Icarus would be cool, and she was an awesome character in Kid Icarus Uprising.
Ghirahim - I dunno, I just have a feeling he will show up randomly. Hey, if the Wii Fit Trainer can come out of freaking nowhere, why not him?
Simon Belmont - Because [tornado fang] you, that's why.
YOUR TURN!