Chance: 65%
This all comes down to whether or not Sakurai cares enough to make Rayman playable. Sure, he has a model in the game, but that's all it is so far. We don't know if Sakurai is actually going to care enough about Rayman to make a moveset from the ground up (I still don't know if Sakurai has seen the publicity of the ArtsyOmni leak). We also don't know the full backstory of why the trophies are in the game in the first place except for the fact that Sakurai himself asked for them. For all we know, he could have just added them for the sake of adding them (though I don't like to think that he did).
I'll say this, Rayman's chances are higher than any other third-party, but it really comes down to Sakurai's Rayman knowledge (if he has any).
Want: 100%
This would be a dream come true for me. I'm saying this not just as a fan of Nintendo, but a fan of gaming as a whole.
Super Mario,Sonic the Hedgehog,Mega Man,Pac-Man,Rayman, and Solid Snake all have a great history across dozens of gaming platforms. The Super Smash Brothers franchise has always been a big family reunion for the popular
and the not-so-popular
video game characters. No matter what you people may think of Rayman, he is a video game character through and through. He also has a 20 year history with the world. Sure, you wouldn't see him on t-shirts as much as Mario, Sonic, and Pac-Man, but he has touched our hearts one way or another with just how zany he is.