True. To be honest, if any wanted character joined Smash, it would be hype. For example, the world would go crazy if K. Rool got added, or if Wolf returned.
Oh yeah, pretty much anyone would start hype, but some characters would cause much larger noise than others. I think most of us would agree that the single biggest possible newcomer announcement, at the very least for first parties, would be Ridley. That would be so beyond insane. K. Rool and really any third party are up there though.
I think quite a few people would say that any more third party characters aren't necessary and they should focus on first party characters, but I think they've already moved pass the "necessary" point after Pac-Man. They made a big thing about having Mario, Sonic, Mega, and Pac, saying they're the pretty much four most iconic gaming characters.
If they were strictly focused on keeping the third party characters to a minimum, they would've left it at those three, but instead they thought it was worth it to expand into other parts of gaming history like the dawn of fighting games with Ryu and the start of the modern RPG with Cloud, so I can definitely see them adding at least one more "historic third party" character on top of Rayman because of Ubisoft's close ties.
Banjo-Kazooie would represent the Rare era of the late SNES and N64, Belmont would represent his iconic platformers of the NES like Mega Man, Shovel Knight would represent Nintendo's current push for indie games on the eShop, etc.
But at the same time I wouldn't be TOO surprised if there aren't any new third parties this time around.