Posting this here and GameFAQs; not counting my vote twice here.
Ridley ~ 80%
Ridley should have been in Brawl as a playable character but was instead relegated to a boss in said game. As a matter of fact, here is the infamous quote from Sakurai that almost everyone in the community knows:
Nintendo Power: There was a rumor at one point about Ridley being playable. Was that ever a consideration?
Sakurai: I think that would probably be pretty impossible. [Laughs] If we had put our best efforts into it, we may have been able to do it. But he might have been a little slow. Would that be all right? [Laughs]
The quote has been taken in many ways. Most Ridley supporters use this quote as evidence that Sakurai sees Ridley as being able to work with enough effort while Ridley detractors sees this as something that indicates that Ridley won't fit in. However, a middling ground is best approached here when discussing this quote; he may change his opinion on Ridley, but he may find it not worth the effort to add Ridley either. However, there are good reasons to believe that Ridley will finally make his playable debut.
First thing to note is that Ridley is one of the most popular characters for this game. He has been among the Top 5 most wanted characters for Smash 4 and among the Top 5 in the West ever since the pre-Brawl days. Another thing to note with his popularity is that Ridley has been highly requested to get into a Smash Bros. game ever since the pre-Melee days. The only other character that was highly popular for Smash 4 that was a popular request ever since the pre-Melee days was Mega Man and he made the cut for Smash 3DS/Wii U. It's logical to assume that Ridley would follow in Mega Man's footstep.
But what is the most important thing that could lead to Ridley's inclusion is that the three newcomers already confirmed (Villager, Mega Man and Wii Fit Trainer) has opened the gate a lot wider for Ridley. The Villager was a previously rejected character that failed to get into Brawl due to his peaceful nature, yet he made an appearance in SSB 3DS/Wii U. Mega Man by being third-party was an incredibly difficult fit, yet he was added to the game as well. Wii Fit Trainer also has a very unconventional and unique style of play.
What these three newcomers show is that if these three can be made playable, Ridley definitely can be made into a playable character. The Villager shows that Sakurai does change his mind on characters he previously rejected, so even if Ridley was rejected back in pre-Brawl, there is room for Sakurai to re-consider him for playable status. Mega Man shows that Sakurai is willing to go through the excruciating work to fit a third-party character in, which Sakurai has mentioned is a grueling process (in some ways, even harder than making the game itself). Being a Nintendo character, Ridley is inevitably going to be a much easier fit even if fitting him in will be difficult; it's still easier than the work it takes to get Mega Man in. Wii Fit Trainer also shows that unconventional style of play can work in Smash and that Ridley could also work with an unconventional style.
What Villager, Mega Man and Wii Fit Trainer have all in common is that they're incredibly unique. They have play styles that can't be emulated by anyone else and they all work out very well. Sakurai is emphasizing a lot on uniqueness (just look at the game play of the three newcomers along with interviews) and Ridley has potentials to be incredibly unique as well.
Put all of these things together and Ridley is a likely candidate for this game. Not guarantee, but certainly up there.
Want ~ 90%
I've been going back and forth in regards to wanting Ridley to be playable, but I've decided to commit support for Ridley. He is long overdue for a spot as a playable character and playing as a space dragon is pretty awesome. This is coming from someone who has never played a game with Ridley on it. Probably would be 100% if that was the case; but I'd be disappointed if Ridley got the shaft once more.
Isaac gets a 57.14% as my prediction. People have become increasingly doubtful of Isaac lately, but there are still a lot of hopefuls left.
Nominate Pac-Man x5.
@God's Robert Cousin, Banjodorf, nessokman: We are not rating Isaac's likelihood today. Instead, we're predicting what Isaac gets tomorrow as a final score.