I personally favor the theory that Sora himself is the M.O.M.
Certainly pretty out there but I think the theory has its merits. Other than that I really don’t know! Anything up for game at this point in time.
Spoilers Alert from KHUX Finale:
It was revealed that the Master has its own origin story. When the Master of Masters was a boy, the Keyblade War, a conflict between light and darkness began. This conflict continued for an unknown period of time, in which the Master saw many of his loved ones perish from darkness overtaking its hearts.
I kinda prefer him to be his own character, turning Sora into a villain would be a completely ridiculous turn for his character, and it would completely annul all the growth he's gone through over the course of the series.
Luckily, there's a good amount of evidence to suggest that this theory isn't true, and I'm much more inclined to believe that MoM is actually some sort of semi-divine entity of light that may even be connected to Kingdom Hearts itself in some way.