The only two true predictions I have are as follows:
#1: It will be two characters that we write off for something that contemporarily makes sense, but in hindsight will look like a ridiculous reason to dismiss a character. It will be celebrated as a busted "fan rule" as if the fans say "you must be this X to be a good pick for Smash", even though we had no reason to expect the trend not to continue.
#2: They will have nothing in common besides minute details with the previous four characters. Console of origin, year of origin are stuff that don't matter, but demographic appeal, franchise, possibly owning company for third parties, possibly genre outside of platformer and RPG, and other stuff like that probably do matter. Inevitably, we'll try to find a way to shoehorn the next two characters into our own pre-existing theories about the final character and the next set of newcomers, be it in Volume 3 or the next console's Smash game.
Aside from that, it could be most things.