How I feel:
It's now or never for Mewtwo and Ridley, we will know their fates by the end of E3, playable or not.
If DK gets a newcomer (which it likely will) then it depends.. If K. Rool, then I 100 % expect an E3 reveal, he'd be too hyped to not reveal before release; if Dixie, then it could honestly go either way.
One of the two swordsmen; either Chrom or Shulk. After the Ike reveal, I feel it could potentially be a setup for a FE newcomer (who would most likely be Chrom); and I could definitely see Shulk revealed to tie in with the inevitable X trailer/info we'll be getting. I just personally don't see two swordsmen being revealed at the same time.
Palutena could again, go 50/50 with me. I mean she's a shoo in at this point, but not sure if they'd keep her secret or not.
Thrid parties; if Namco gets a rep (Pac-man) then that rep will be revealed here, no doubt. They haven't even received credit on the official website yet, so I feel like they're waiting for their rep to be announced to put their name on the site. Also, either Snake or some other Konami rep (Simon Belmont anyone?) Will definitely be revealed, if Konami does still in fact have a rep. It's pretty much guaranteed that all remaining third parties will be revealed at E3.