I called it ( Final boss of endwalker)

[RitsukoSonoda]

I wouldn't be surprised at all if the final boss, whatever it ends up being, takes on the name "Zeromus."

There just isn't any evidence for it at the moment.

Given Zeromus's lore in FF4 if it does make an appearance I would think it would be something that Falafel (or whatever that guy's name was, I remember it starting with f) would turn into in some way.