That's not my guesses.
The two third party characters that come to mind, is the rumored Simon Beaumont from Castlevania, as that would finally get all of the classic JP developer's into Smash (Capcom = MM/Ryu, Bandai = Pac, SEGA = Sonic, Square = Cloud), and the other, is Steve/Alex from Minecraft.
Why Minecraft, is simple, the seeming relationship with Microsoft and Nintendo being more clearer, especially seen when we look at cross-play from Minecraft, Rocket League, and a few other titles as of now. And, that I think it'd be a possible sign of good sportsmanship that both companies seem to sometimes have. Not to mention, that would be remarkably easy for Nintendo/Bandai to have, as Steve/Alex can be interchangeable like the Villager, and have a variety of attacks.