I don't know, I personally feel that it's too late at this point. Unless he gets revealed by the next Nintendo Direct I can't really see him happening at this stage.
I think Sakurai just wants the three legendary mascots together with Mario first before he includes any other third parties for this game at least. I'm not very sure if Snake will return either, and if he doesn't, I can see the fans pulling a Mewtwo and pushing more for him to be reincluded in the next games more than any other third party.
Might just be the pessimist in me talking though.
Would love to see both Layton and Snake confirmed at the next Direct but I'm not going to cross my fingers for it.... (at least for Layton)