I still think Spirits Deconfirm and that person was saying it for clout, but if it is real, I think it's Geno and Rayman. Both are highly requested, Sakurai wants Geno, and why get a company as big as Ubisoft only for a collectible twice in a row? Elma has a chance, but the fact that she's Nintendo holds her back quite a bit, Shantae is too obscure (that's coming from I'm a pessimist who wants her in), so I can't see her getting in, especially with Shovel Knight and sans deconfirmed, as well as rumors about Reimu taking the potential indie fighter slot.
As for Bandana Dee, he's got a ton of things holding him back. Firstly, he's Nintendo, which is a big DLC no-no (or at least only one per game), he's 95% Modern Kirby, which is a big Smash Bros no-no, and his world is already represented with a fighter, which means that Sakurai would turn him down even without bias. (It doesn't really apply to Elma since there's like only one tiny reference to XB1 in XBX)
Edit: forgot to mention this: People HATE Bandana Dee. For every one person who wants Bandana Dee, there are two people who think he's a hat goomba, which probably includes Sakurai and his team, considering how they added Bandana Dee's spirit as an enhancement to normal waddle dee, when the slot could have been held by Parasol waddle dee