Excuse me whilst I barf
Rayman is still pretty iconic character in his own right, and he would fit in much better with the cast. Even if Ubisoft doesn’t see Rayman as a mascot, he is easily the most requested character from that company, and I doubt Sakurai would pass over fan demand for a character from AC.
And really? You think Ubisoft not using Rayman as a mascot would seriously hurt his chances? Did you forget that Smash Bros brought in Mega Man in 2013, a period where Capcom had all but forgotten he existed?