Given the fact that Smash Ultimate's project plan was finalised in December 2015, that most likely means Sakurai did not know about Mario + Rabbids.
This potentially means that the Rabbids, along with Spring Man and Rex & Pyra, came along too late to be considered for the base game. Granted, the Rabbids have been a successful franchise since the Brawl days, but the demand for the Rabbids to appear only started to grow with Mario + Rabbids due to it being a high profile Switch exclusive. However, the game came out in August 2017, which was about a year and a half after Ultimate's finalization in December 2015. At this rate, the Rabbids could make an appearance as an AT unless if Sakurai wants to make them a DLC fighter.
What does this timing mean for Rayman, then? Well, a lot, actually.
For starters, Rayman Legends was still a pretty recent game as it released in September 2013. It was one of the few high profile third party games on the Wii U. Additionally, you could make an argument that Rayman's popularity was at its peak during the Ballot era due to the three trophies in Smash 4 and the fake Omni leak. Whether that popularity would get him in over other candidates is entirely debatable since none of us know the top requested characters on the Ballot other than Cloud, Bayonetta, and maybe Snake. We also don't know how popular Rayman really is outside of Smashboards, but you can say the same thing about every other potential newcomer.
As for Sakurai not knowing about Ubisoft's extensive support, I don't really think that's an issue. Besides the fact that Ubisoft was the biggest supporter of the Wii U, you could argue that this aspect does not matter much for a character's inclusion since Konami did not really support the Wii U and 3DS back then. Despite this, though, not only did we get Snake back (as well as David Hayter!), but if the leaks are to be believed, then Simon Belmont may very well happen.
By no means is Rayman a guarantee. His biggest obstacle is the lack of Japanese popularity. However, Rayman already has his foot in the door with three trophies as well as decent popularity. As a result, I do think the timing of this project plan bodes well for him.
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