Okay, after watching videos on youtube about the newest secret endings of 2.5 HD remix, I think things are starting to have a thread to connect them together;
First off, Young Xehanort and Braig were talking about the oldest keyblade that has been passed from Masters to Apprentices in an almost seemingly far away time, which is the one Master Xehanort inherited.
Which can be associated with KHX which mentions about Foreteller's tome of prophecy and the 6 apprentices who became the foretellers of their own unions. And in my own thinkings, It seems convinced to me that Master Xehanort is the successor of the 6th apprentice who never shown up or forming any unions, or being bestowed with one of the tome of prophecy from their master;
1) They all have similar keyblades which have signs of animals and tokens of eyes, so its pretty obvious that the keyblade Master Xehanort wields is formerly being wielded by the 6th apprentice.
2) The 6th apprentice never shown up or had the responsibility of keeping the prophecy safe, and guarding the light from invasion of the darkness. So it can be assumed that he (the 6th apprentice) is someone their master didn't trust.
3) The foretellers did give out a warning of someone disguising himself as a guardian of light, but seeking the light for own greeds.
4) The uncanny association of number 6 and goat sign on the keyblade.
And thats where things get interesting, considering how the keyblade war was ended, I'm assuming that the 6th apprentice had done something to wipe out the other 5 apprentices and managed to collect all the tomes of prophecy, which was inherited by his followers along with his will and keyblade, which was inherited eventually by Master Xehanort; as Young Xehanort said, he hadn't have obtained that keyblade yet, pretty much means that when he gets back to his time and lives the life fate has in store for him, he will grows into the exact person who has inherited all these stuffs, and the passed down will.
And one more thing, the part where Maleficent found out the connection between world data and foretellers' texts must have been planned by Master Xehanort, as he used her to kidnap Minnie in the most convenient moment in KH3D.
(p.s: Young Xehanort was asking Braig to take Isa with them to do the time traveling, as he's important to the True Organization XIII)