When comparing Rayman to the third parties who have yet to make a Smash appearance, Rayman seems to be the most likely third party newcomer.
Let me give you an idea as to how likely the other third parties can get in Smash:
Simon Belmont: Although considered a retro gaming icon, he isn't really relevant and Snake already represents Konami.
Bomberman: Faces the same obstacles as Simon, pretty much.
Bayonetta: Although her latest game was exclusive to the Wii U, Bayonetta isn't really known by the masses and coming from an M rated franchise doesn't help either.
Ryu: Although a fighting character at heart and having history with Nintendo does help, Mega Man is already representing Capcom.
I don't think we need any more third party characters beyond Sonic, Snake, Pac-Man, Mega Man, and potentially Rayman.
After all, they're mascots of gaming companies that have influenced gaming one way or another.
Personally, I would be satisfied with those five.