(This theory is about Master Xehanort, which I will be simply stating as Xehanort.) My theory is that prior to Xehanort having the same master as Eraqus, he had another master who was originally a vessel of the 6th foreteller. Perhaps Xehanort's original master had passed the keyblade to him forcefully and erased his memory but leaving some hints of the keyblade he passed on. After that, Xehanort would be found by Eraqus's master and they would train together. Maybe Xehanort's original master will be the antagonist of the next saga?
Evidence is that Xehanort wonders what happened during the keyblade war and will cause another just to see what would happen.