Geno (everybody expects another 3rd party game crossover, he'll make ALL the nostalgic battle sounds, Mallow helps him attack or Final Smash, Exor shows up somehow).
Goku (but it's original Kid Goku, which is why FighterZ ended up with Kid Goku GT. This means his power set is a lot more in-line with Smash, more use of his pole and cloud, and it harkens back to just how far back Dragon Ball has a history with Nintendo. People will gripe about not having Adult Goku, but he's not a great fit, and trying to limit him would end up with... Fast Flying Ryu? Rainbow Edition Ryu.)
Bonus: They'll make Waluigi happen. They'll play up the whole "left out" angle. He'll probably be the absolute last character released, although there's a small chance Nintendo might have a smaller team make 1 or 2 extra, free characters later on during the Switch's life, possibly as major cross-promotion for something. A new Pokémon, some revitalized IP, hard to say.
Shame Google Theory might've spoiled the remaining two. Honestly, I don't really want the Doom Marine; they've probably got a way to make him work well and it'll be awesome (first-person shooting during Final Smash, that won't make other people queasy), but I'm just having a hard time seeing the character be anything but shock value and "Oh that's neat I guess."