So I guess to me, the most likely Nintendo characters are Kiran, Alphonse, Sharina, Anna, Hyness, Bandana Waddle Dee, Someone from Hyrule Warriors: Age of Calamity, Decidueye, Samurott, Typhlosion, or Arceus.
I'm pretty sure that if they wanted to promote Kirby Star Allies specifically, they would go for the Three Mage Sisters; Hyness is a bit too much niche and was initially considered to be playable (data of a playable Hyness were found by datamining) but at the end they went with the Three Mage Sisters in the original game already, so I think they would do the same for Smash.
Also I don't think Bandana Waddle Dee would really work at promoting the game, he's not that important in it, regardless of what people say.
Kirby Star Allies' most iconic characters that would work as shill are the Three Mage Sisters or some of the most exotic Dream Friends, which Bandana Waddle Dee is not one.
On top of that, of the Dream Friends, I would exclude:
-Marx, because he's already a boss
-Gooey, because if he was planned to be a fighter they wouldn't have given one of his signature attacks to Kazuya
-Dark Meta Knight because, despite having potential of being completely unique, I don't see him joining Smash in any other way than as an echo or a skin
So, Imo if it's a character to promote Star Allies, it's either one among Rick-Kine-Coo, Adeleine&Ribbon, Daroach, Magolor, Taranza and Susie, or the Three Mage Sisters, at least IMO. (I would love any of them BTW)