I'm like half a day late on this, but as a big Rayman fan (top 3 most wanted since brawl) I'll explain why I personally don't have much remaining hope.
For starters, you'd think it'd have happened already. More than a couple games on Nintendo platforms, then to top that list with the Rabbids crossover, I thought for sure he was coming to smash. Especially with how odd I thought it was that they didn't use the chance to make Rayman and Mario meet up.
Also around sm4shs development, Michel Ancel, who created Rayman, was asked in an interview "Can Rayman kick Mario's ass?" and he responded "Uh, maybe we'll have the answer very soon."
Ubisoft was originally going to timed-exclusive that game for the Wii U, but decided to halt its release to work on other consoles' versions first, for a full mult-plat release.
I think Nintendo took that really personally, especially with how much the console struggled. I'd be willing to bet Rayman was supposed to be in Sm4sh, and was pulled.
Ubisoft making an appearance in costumes did restore a little hope, on that maybe they've made amends, but putting yourself in Sakurais shoes: if you had spent any real amount of time trying to work a character into your game just to have to shelve it however far along in development, knowing that time could have been spent on someone else, it'd probably leave a bad taste in your mouth and make you not want to revisit that character.
That's just how I interpret things surrounding him.