Who would really be another Animal Crossing representative, though? F-Zero, Bayonetta, and Xenoblade are easy to do, but nearly everything Villager does is representative of the entire Animal Crossing franchise. Unless you want Pete the Post Pelican whacking people over the head with his mailbag.
Don't think they'll be another Animal Crossing character, but if there was to be one then it'd probably be Isabelle since she's in MK8 too. Tom Nook is pretty iconic as well.
Doubt we'll get another F-Zero character unless a new F-Zero game is announced soon. Maybe Blood Falcon as an echo fighter, would be an easy addition.
The number of Fire Emblem characters is a bit excessive and almost makes me think they wouldn't dare add another...but they'll probably do it. With so many characters in the roster their overall presence will be slightly diluted too.
Can see them adding Jeanne from Bayo, maybe as an echo fighter.
Personally would like to see Travis Touchdown, Dante, Nero and a Castlevania rep like Simon Belmont.
Isaac from Golden Sun could be a possibility for a new character - he did fairly well in the character poll didn't he? Other than that, can't really think of many new characters I'd add. Incineroar or Decidueye seem the likely candidates if they want a Gen 7 Pokemon (Decidueye possibly more likely since they have its Pokken moveset to riff off, but Incineroar would be cool as a grapple-based fighter). Maybe Zelda will get another rep, feel like they have a fair few options with the Champions, Impa, Midna etc. to go off (Impa could be a cool Sheik echo perhaps).
Also yay Ike's eruption goes forwards now, my boi getting new tricks
I think it does it make unlikely tbh. Would be jarring to have a playable Midna appear in the same match as an assist trophy version of her. I guess they could make it so Midna doesn't appear as an assist trophy if anyone is playing as Midna, or they could just remove Midna as an assist trophy, but this is all pretty finnicky (and they did already bring attention to her as a new edition in the e3 announcement).
I also don't believe an assist trophy character made the leap to being a playable character in Smash 4.
Like I said though, I don't think it's impossible, just very unlikely unless there's a sudden surge in Midna popularity like with Ridley.