I'm still not on board on the idea that Pyrosphere will be a neutral stage. I think it's very likely that we're going to get some form of hazard on the stage, whether it should be lava or a boss hazard.
However, I don't think Ridley will be that boss hazard. There's probably going to be some bait-and-switch trick on us and we'll get Rhedogian instead.
The stage is literally Pyrosphere that Samus fights Roidley in. If there's a stage hazard, Ridley isn't just obvious, but pretty much the only logical one. I don't think they'd leave people speculating like this for a stage hazard, but if it's a boss hazard in that level it's almost certainly Ridley or a type of Ridley.
You never know, they may have made two Ridleys, in a reference to the battle in Other M in Pyrosphere being against a clone, not the actual Ridley...
...oh boy wouldn't that be fun. The trailer starts, some characters is in Pyrosphere, battling Samus, only for Roidley to show up. People are disappointed, is that really the Ridley they chose? One of the characters is a newcomer, the splash screen comes up for them, but not Roidley...
Panic starts to set in the audience, they show a short battle, the voice over comes on and directly confirms that Ridley is the stage hazard here, references Wily Castle as another example, the internet is literally in tears at this point.
When suddenly, from the edge of the screen, who's this? A purple blur comes in, a splash screen appears: "Ridley, bigs himself up!"
It happens, Ridley is playable, and he curb stomps Roidley, Samus, and that other newcomer, probably Shulk, but who cares, Ridley!
I mean, it could be another Pyrosphere enemy there too, but I doubt they'd be bothered. They'd have gone for Pyrosphere for Ridley.