I'm surprised Rayman isn't being brought up a lot more. Nintendo has been partnering with Ubisoft a lot for several years now. It would make sense for Nintendo to want a Ubisoft character as DLC. They even published Rayman Legends in Japan, and added 3 Rayman trophies in Smash Wii U. There are also Rabbid spirits in Ultimate, so we know they've worked with Ubisoft on this game already, even if it is to a minor extent. Not to mention Rayman is a popular character with a notable amount of fan requests.
He may not be popular in Japan, but people are already willing to accept that a Japan only 3rd party character might potentially get in.