Might be too late. Oh well.
But I'll get it out of the way and say
Want: 100%
Ridley would be one of my top characters to add, at least on the basis of the character - some like Wii Fit Trainer I would never have rated that high, but I thought the move set looked interesting and the idea was a bit crazy and I really enjoyed the addition despite her not being on my list at all. I like the Metroid games a lot and there aren't that many characters to choose from, and Ridley is pretty much the most important character aside from Samus and Mother Brain (who doesn't work as well as a character given that her iconic appearance is "brain in a jar"). Dark Samus is possible, but seems to be done (pending more information about Metroid Prime 4! Not that Ridley won't likely make an appearance in that too). So anyway, Ridley would be the natural next Metroid character. I also would like having more villains.
BUT
Sakurai has expressed on multiple occasions that he doesn't think Ridley would work. They put him in Smash 4 as a stage element because they thought it important that he be massive. I did not enjoy it and did not find it the perfect way to honor him or whatever. I would avoid playing on that stage due to Ridley, which means instead of me enjoying his inclusion, it's actively negative. Maybe it's not for other players who are less competitive/enjoy more randomness.
HOWEVER, if they're not adding many characters, Ridley would be a relatively high bang for the buck addition. Sakurai and Nintendo also know that Ridley is one of the most wanted characters in the US. They have added characters that were previously ruled out, and Sakurai seemed to imply he didn't want to make another Smash but Nintendo is evidently making him do so anyway. So Sakurai's statements don't make him impossible. It might complicate including the Pyrosphere, but honestly, I wouldn't mind if it was cut, or Ridley as a stage element was cut. (They could include a Samus Returns or Metroid Prime 4 stage instead, perhaps...)
Ridley chances: 10%
The chances are stacked against him, but it's undeniable that he would be a high impact, hype-building inclusion. With Samus Returns including him as the final boss and Metroid Prime 4 in development, promoting Metroid wouldn't be a crazy idea...
Nominations:
Simon Belmont x5