Rayman has been Ubisoft’s corporate mascot since his mid-90s début, although other Ubisoft properties (such as the Assassin’s Creed and Tom Clancy franchises) have eclipsed his popularity in recent years. Rayman has been a presence on Nintendo consoles since Rayman 2: The Great Escape, the 1999 critically acclaimed platformer, appeared on the Nintendo 64.
More recently, Nintendo let Ubisoft develop and publish the surprise hit Mario + Rabbids Kingdom Battle, a tactics-based RPG crossover featuring Nintendo’s mascot and the Rabbids (spin-off characters that first appeared in Rayman Raving Rabbids, thus making them technically a part of the Rayman universe). Mario + Rabbids Kingdom Battle is also, at the moment, the top-selling third-party game on the Switch.
Although Rayman isn’t currently a playable fighter, he’s already been implemented in the Smash Universe. Rayman first appeared as a collectible trophy in Super Smash Bros. for Wii U and 3DS (likely as a tie-in to Rayman Legends) back in 2014. Rayman is also already in Super Smash Bros. Ultimate as a spirit card, meaning that there is obviously no issues regarding the character’s license. Furthermore, Mario + Rabbids Kingdom Battle has sold well over two million copies, which is significant if you consider that Byleth’s addition to the roster was likely due to Fire Emblem: Three Houses also being more successful than expected.
Even though some fans will argue that Ezio Auditore from Assassin’s Creed II is more popular than Rayman, I will argue that the Assassin’s Creed franchise does not “tonally fit” within the Smash Universe. To be honest, I would be shocked if Rayman wasn’t one of the last six fighters.