When I first thought about what third-party characters might join SSB4, I imagined Rayman, but then quickly dismissed the ide--that Ubi isn't such a "legendary" publisher like Konami, Sega, Capcom, Square-Enix, or Namco-Bandai. Similarly, Rayman doesn't hold the same weight as Snake, Sonic, Mega-Man, FF/DQ series, or Pac-man.
...The more I think about it, the less crazy it seems.
Ubi's been a huge supporter of Nintendo's recent platforms, especially with the launch of Wii U (ACIII, Zombi U, Rayman Legends, et. al.).
Additionally, Nintendo plans to publish Rayman Legends in Japan, which should drum up more sales than the game may have seen had Ubi published it.
There seems to be a strong relationship between the two companies.
Honestly, I'm half-expecting this to happen...