Wish I could cling to that hope that N has a notable chance, but he really doesn't at this point. Unless he appears in XY. Dunno. Even if he did, I'd kind of doubt he'd get in.
While I don't think that Newtwo is an extremely likely canditate, he's more likely then any other Pokemon character at this point. Thus,
If Newtwo gets in, I'm sure Mewtwo will be in too. The demand for him can't simply be ignored if a f*cking form of him gets in. It would probably switch forms via Down B, or via Final Smash. I can't really think of any other likely way to change form mid battle.