The only way I could see an assist trophy in Ultimate beeing upgraded to a playable fighter is, if we get a smash ballot, and if these AT character have a lot of votes...
As much as people make fun of some Waluigi fans, if they still ask for him and stay active, they could make the difference in a Smash ballot.
However, I don't think the new AT of Ultimate could get this treatment. If Bomberman is an assist, I think that's because they were candidates with "higher priority", especially in the third party realm of characters.
But character that were AT since Sm4sh/Brawl and are still AT in Ultimate COULD be upgraded to playable fighters with a Smash Ballot in my opinion.
But that's pretty much more of a theory than anything, and even if we got a Smash ballot for Ultimate, there will probably be new characters that people will vote for.
Also, I'm pretty sure this has already been talked about but... If I remember correctly, during the pre-release Sm4sh era, in one miiverse post, Sakurai said that he contacted Ubisoft to get the rights/concept art(I'm not sure about that tbh) of Rayman, and Ubisoft agreed and send them 3D models of Rayman, Globox and Barbara for their trophies. Sakurai was surprised to get those models if I remember correctly.
The fact that Ubisoft had 3D models of Rayman,Globox and Barbara in the first place still surprise me, since Legends and Origins were 2D games. And the Rayman/Globox models are too different from Rayman 2/3, they have the Legends design in 3D. I don't think they used it for marketing purpose, so that pretty much mean that they did it for Smash, even tho Sakurai just only ask to get the rights for the characters.
So, I'm pretty sure it mean that Ubisoft Montpellier would be more than happy to see the limbless wonder in Smash. I know it's hard to prove if third party companies want their characters in Smash, but I think Ubisoft would be willing to give the rights and even help Sakurai and the Smash dev team to create Rayman in Smash. Ubisoft Montpellier and Michel Ancel have a real passion for video games , and if they can get a character in Smash, they know that Rayman is the best candidate ; just look at Brawlhalla, they could have decide to use any other character, even the rabbids, but they went with Rayman.
So, it's pretty much up to Sakurai right now. I'm still sure he's going to be DLC, since we already have a new third party in the base game (
) and since we're getting less characters, I don't think we're going to get Rayman now, especially with the very possible SE rep (Geno, Sora, a DQ Character, ect...).
Or maybe he's going to be an assist and is held on for the last Smash direct.
If he is... I hope he get the Alucard treatment, and be a assist with 3 differents way of attacking/moving, ect... So he stand out a bit much than the others.
I just want him to get more love, so we have more Rayman games. Getting in as a playable character would be a dream come true, but if he's in Ultimate in any shape of form, I would be happy !