I have to wonder if the "hints" towards Doom Guy are legit. Smash character reveals are such a big thing, and I'm not sure Nintendo would be chill with at least some Bethesda guy saying "oh we might have a Nintendo/Dewm surprise coming up wink wink nudge nudge" (though they did let a box design accurately point towards the number of remaining newcomers for Ultimate, one of which would be placed somewhere near Ryu as an echo). Surely Nintendo would have NDAs in place to if anything, keep certain discussion of the product from happening or whatever? I mean the closest we got out of Rareware was "if that were to happen, we wouldn't be the ones to announce it", but then we'd get Kirkhope deliberately denying Banjo stuff for E3 with "don't hold your breath", right?
Anyways, though not a newcomer, it'd be pretty neat to see some FP characters get returning mode things, if not all of them. Mixing stage builder with mini-games seems like a no-brainer, just look at the character-specific obstacle course custom stages people have made. And it'd fix issues with making individual stages for 80+ characters.
And as said, I don't know if we could do too many more bosses before running into a really long mode run like this game's hypothetical All-Star mode. 10 bosses already, 8 if we doubled up with the hands and eyeballs. Add a Classic Mode mural setting and a minigame for that Master Hand thing and you're golden.
And instead of adding more bosses, they could add in some more enemies to redo Smash Run in addition to a bunch of existing KO-able, non-timed ATs and the Ditto Pokemon ball (and of course, Piranha Plant fighters, maybe...). They didn't have a department for that stuff when making the base game, hence why we got that adventure mode, but they could make some good use out of the ATs. Don't know if technically you could do stuff like, splitscreen TV mode runs, map interactions between players, but you could at least customise it more.