Ezio made it int' Soul Calibur, so it's not at all Farfetch'd t' say Connor has a chance of gettin' into SSB4 (which would be ****in' AMAZING ). If fan demand is high enough, it's very possible he'll make it in as the next third party character(s)... if Ninty adds any more. Which is hilariously unlikely in it's own right, 'course. =/
That aside, the only thing that could hurt the possibility of an Assassin's Creed character gettin' in is that no single AC character has been in two Ninty games (with the exception of Desmond, if he was in that shitty DS AC:I prequel). Connor would have t' break the character popularity charts, which I doubt since everyone who's a fanboy of AC tends t' get all girly-eyed for Ezio... And there's obviously no way Ezio would find his way in since he's already made a cameo and is an older AC character.