Ridley is hard to discuss from an argumentative standpoint. From a standpoint of inferring from things we've already seen, it's easy to conclude that Ridley is probably going to be a boss, as indicated by Sakurai strongly hinting at that notion in the Direct.
But honestly, I don't know exactly why Ridley should or should not be playable, or which role would be best for him to take in the game. I'm leaning towards him being a boss, not just because of evidence but because I think that role best fits him. I just know these things:
1. Ridley has always been a boss in the Metroid series.
2. Ridley has always been a boss/hazard outside the Metroid series when he's not just an SSBM trophy (Nintendoland, Brawl, Dead or Alive).
3. Save for his size in NES Metroid, which is due to Samus' short firing range, he has always been large. Changing sizes, but always large, and several times larger than Samus.
4. Even though he's small in comparison to other Metroid bosses like Kraid or Phantoon, he is still large.
5. His gargantuan size is not his most important characteristic but it is a consistent one. He has been able to intimidate Samus, as well as harm her and grab her as if she was an action figure in comparison to him. No doubt size could be partly attributed to his ability to do that.
6. Ridley has consistently been able to come back from the dead in some form, to Samus' surprise.
I mean, these are just facts. I really don't think it'd be best for him to be playable. You could say that Ganondorf has always been a big bad boss that's has been able to constantly come back from the dead. But think about it. In Metroid, it's just yourself, and maybe sometimes some outside help (i.e. GF, 07th Platoon, The Baby, etc.) but you're really out by yourself and for yourself. In Zelda, it's you (Link) and a whole slew of characters actively willing to help you out (Fairies, Zelda, Midna, Fi, various townsfolk, etc.). You're on an adventure, and you can be separated from other friends at home and other places, but you're never alone. Samus is alone. She's got Anthony and the GF, the latter of which she cannot wholly trust anymore due to the events in Other M and Fusion. Every other personal friend/helper of hers is dead. Not only is she alone, but she has an enemy (Ridley) that keeps coming back from the dead to kill her.
I don't really know. It just doesn't seem right for Ridley to be playable, although I could be wrong. I could try to elaborate more but it isn't really worth it, haha. We'll just have to see what happens in the upcoming months!