If Ridley gets in he'll probably be horribly butchered. I can't see any conceivable way for them to get Ridley to work in the game without defeating the purpose of having him in the game. He worked far better as a boss fight, because a boss fight allows for a Ridley appearance that actually manages to be like Ridley. Hell, they even gave us two boss fights, which to me seems like they're trying to make up for the fact that Samus is still the only Metroid character.
One thing I want to point out, the "Ridley is too big" argument has nothing to do with his size limiting him, it's the fact that one of the main points of Ridley is that he's a giant dragon. If they take away the "giant" part then it kind of loses the point. As a boss fight, they have no reason to reduce his size, so it all works out.
Hell, let's create a character here. Say, "Fred the giant." Fred the giant stars in a very popular game, takes off, and before you know it people are requesting him for Smash. However, there's a problem - Fred the giant is, well, giant! There'd be no way to include him, since if Fred the Giant was reduced to standard size, then he'd no longer be Fred the Giant. Just Fred. And no, I'm not saying his size is his only point of appeal, but it's still a major part of it.
And as someone else pointed out I think, Ridley is pretty much a cinematic boss fight. He has large, grandiose attacks that wouldn't work in a fighter unless they were toned down a lot which again, would be significantly less awesome. Again, this is why I like Brawl's boss fight - I'm able to fangasm over Ridley without feeling disappointed that this was what we got.
Sakurai could still put Ridley in the game - the demand is there, and the concept has been brought to his attention. It's still a possibility, especially considering the lack of Metroid representatives. However, he just isn't a good choice as a fighter.
I'm probably gonna be called a troll and be told to go back to Gamefaqs for daring to speak my mind on this matter, and judging by the hundred pages of debates/arguments/****-slinging in this thread I'm obviously not going to accomplish anything here. Just venting my thoughts here, so don't expect me to argue any more beyond this.