Honestly, after getting King K. Rool, I just want to see Rayman appear as anything but a trophy.
I have a feeling Sakurai is going to do something for Rayman or Ubisoft in general. So far, all of the newcomers (K. Rool, Chrom, Ridley) and Assist Trophies (Shovel Knight, Zero, Krystal, Knuckles, Bomberman) have been characters that people have requested for and Sakurai might be going out of his way to putting in as many popular characters as possible, whether as a newcomer or an AT.
Rayman is a weird case, though.
I would have been positive about Rayman appearing as an AT if it weren't for Sukapon.
I mentioned this before, but the thing that really bugs me is that there are very few characters like Sukapon and Rayman and there are even fewer that have ties to Nintendo.
Another thing worth mentioning is that each of the new ATs seem to be replacing old characters that might become playable (Shadow>Knuckles, Isabelle>Kappn, Moon>Skull Kid, etc) and have unique functionality such as Nikki, Klaptrap, etc.
Now, I'm not ruling out the possibility of Rayman being an AT. He could be like Sukapon, but attack in the air and more agile. Considering that this is Sukapon's first appearance as an AT, though, I'd question the possibility of Sakurai making Rayman an AT when there's already a character like him already.
Am I grasping at straws? Maybe. I think it's a possibility worth entertaining, though.