I think that all the characters that were once in Smash Bros but were scrapped (Roy, Mewtwo, Pichu, Young Link, Dr. Mario, and maybe others that I'm forgetting) should be Assist Trophies. Well, except for Pichu and Mewtwo, who would be Poke Ball characters.
Batman is not so awkward as you think
And he is coming to the WiiU
I thought Snake was an awkward choice yet he fit in nicely
Masked Link is also not that bad and is somewhat likely to get in (Especially if they make a 3D version of it)
Ephraim is also likely because he is non-sword user and a lord. Plus he is in a DLC for FE: Awakening
(Which also has Roy and Lyn and others)