I agree with the posts that say we are certainly through with returning stages. We might see DLC for some of them but I doubt it.
The only thing up in the air is if they're going to reach that 106 non-Smash original mark, the max the CSS as we see it can hold. Sakurai doesn't leave false hope, and this would be the first major case of that being a thing. He didn't leave it for Skull Kid, he didn't intend to with Ridley (he had a shadow on Pyrosphere presumably because he wasn't ready, not to tease people who wanted him), and the previous time with the Ice Climbers was literally out of his capabilities of fixing. This would be the first, unprecedented time Sakurai has left behind false hope when he could just outright say "The remaining fighters will come from franchises that we already have in the series. We apologize if you wanted a fighter from a new series, but they can still join the battle as an Assist Trophy!".
This could be because he's uncomfortable admitting it. It could also be that we aren't done with new stages. We could be, but we could still have homeless newcomers until the DLC season. By the way, my most wanted probably doesn't need a stage THAT much (despite being a third party) since there aren't that many significant locales from her series, and could easily just make do elsewhere until DLC season, but my other ones do need stages (except Paper Mario). R.O.B isn't the only case of this: F-Zero and Mother in 64 and Fire Emblem in Melee also have been homeless. Space/time limitations, yes, but that's still a thing.
As for Yokai Watch character (Jibanyan presumably), I think it's possible, but not that likely. Much like how Arle has to fight a second Sonic rep, Jibanyan has to take on Layton, which would cause split votes between the two of them. In fact, that's happening a lot. Geno vs. DQ Rep vs. FF rep 2, Banjo vs. Steve?, and Lloyd vs. Yuri vs. Heihachi.