There were two others, right? Rhedogian and something something I don't know. How would they fare?
Is there also the possibility of an older enemy appearing as a boss?
Well with Asborean, it attacks by sliding around along the ground, trying to ram into Samus, shooting smaller Asborean Spawns that home in clusters, and by getting up onto its hind legs and trying to headbutt or swipe. I can't really see it fare well on Pyrosphere to be honest. Rhedogian on the other hand is more promising. According to the wiki, it can "fire a spray of homing missiles, use its arms to propel itself around or sweep them against enemies, have the
ability of flight, can ram enemies and can charge and fire a massive Zero Laser-like weapon from a singular eye. It is also capable of grabbing foes to slam them into the floor. Sometimes it will suck in a vortex of air before firing the beam". The ability of flight, it's not just Ridley that has this ability, another Pyrosphere boss does as well. What's more interesting, during a cutscene with Samus and Anthony Higgs, the Rhedogian that they encountered escapes by
jumping into the lava, so technically we have
three bosses that can interact with lava and allow Ridley to be a playable character and not a boss on Pyrosphere. I can't see retro bosses (Crocomire, Phantoon, Nightmare, etc) get in, but I wouldn't say it's impossible.