So as you probably know, the leaked roster had what appeared to be an incomplete roster with five open spots for characters. There have been several "leaks" that are only text stating who the characters are. However, like the DLC leak, it was just the leaker of the first screenshots using his newly gained persuasive power. I therefore regard all text leaks false, leaving the spots open. Here are my predictions.
Ice Climbers[]
Having been in since Melee, the Ice Climbers are classic NES characters. They are pretty popular with fans, and I would be slightly surprised if one of these five slots didn't go to them.
Impa[]
With a roster much larger than I expected, I think Zelda definitely deserves a sixth representative. Impa is a commonly recurring character in the Zelda series, and while she hasn't actually engaged in combat all that much in the series (not including Hyrule Warriors since that's too recent), with some creative thinking she could be given an interesting and unique moveset.
Mewtwo[]
Dr. Mario's return has shown that it's not impossible for members of the "lost five" to return. Mewtwo is the probably the most likely character in the unconfirmed four to return, being the only one who's not a clone. Just because Shadow Ball is similar to Aura Sphere doesn't make him a clone of Lucario. Plus, Mewtwo had a very recent appearance in X & Y, having his first appearance in a Main-series Pokemon game since 3rd Gen. I didn't think at first that Pokemon could have six characters, but now that we're getting such a large roster, I think it's possible.
Wolf[]
While I was hoping Krystal would replace Wolf, one of the screenshots in the leak showed Krystal's trophy, which didn't really look like a fighter pose. While Wolf is a clone, I don't think Starfox should go down to two characters, and Nintendo could always change his moveset a bit.
?[]
I honestly have no idea who the fifth character might be, plus I was in kind of a hurry while writing this, and I couldn't decide on a character.
These are my predictions, write yours in the comments!