Notes:

So during the April direct, during the part where the boss characters where shown, Ridley's wing appeared at the 1:02 mark in the video of the section of the direct here https://www.youtube.com/watch?v=fb35vh33Mb8, but it was grey. It might just be an unfinished model, but similar to how Alfonzo takes the role of Toon Link on the Spirit Train when he is played, Meta Ridley might fill the boss role on the Pyrosphere if Ridley were to be playable. If not Meta Ridley, since it's the same creature as the original Ridley, Metroid: Other M's clone Ridley could be a boss, while the original Ridley could be playable.