I'm not wholly banking on this line of thought, but, considering that Sakurai sometimes defies convention with new additions to the roster, wouldn't it be something wacky if Geno and Mallow both got in as the only two characters who had the honor of sharing a character slot with different moves, switching between the two of them as if they were different skins? (Four for Geno, four for Mallow)
I absolutely do not take much stock in this, because I don't even think the game would be able to allow something like that. I merely just think that, even if we get Geno, it'll feel slightly weird that we would have nearly every playable character from Super Mario RPG (Mario, Peach, Bowser, Geno), but Mallow's just kind of stuck potentially being a head on a Mii Fighter.
There is zero chance that Sakurai would devote two fighter's pass slots to two characters from the exact same game, so, the only other way I could think for him to skirt past that issue would be to stack the characters on top of each other in the same slot. And yet, Geno and Mallow are too different to be considered an Olimar/Alph situation, so that wouldn't necessarily be the way to do it, either. But, again, this is me just kind of throwing words out; if Geno is in, I have incredible doubts that Mallow would be in right along with him, despite how much I would prefer otherwise.
I guess it's just fun to consider what kind of breaking-of-conventions could be done in order to throw the Mallow crew a bone, too.