I have various thoughts:
+ Master Xehanort: Is Xehanort the Sixth Foreteller and has been acting to not know that much about the Keyblade War for plot's sake or has Xehanort been a willing vessel for the Sixth Foreteller? What if the Sixth Foreteller has been concealing his appearance (or it's actually his true appearance) as the Guardian?
+ Master Eraqus/Terra: What if Master Xehanort has them under his control because they know his secret plan regarding the Foretellers? They were or are in his heart (or close to that of the Sixth Foreteller).
+ Young Xehanort: Assuming he betrays Master Xehanort; why would he, though? Assuming the Sixth Foreteller is a separate entity to MX; is the Sixth Foreteller possessing YX right now? But why would he posses YX? Does he think that, as a younger vessel, he can exploit his powers more?
+ Chi: What if the world of Chi is actually a digital recreation made by Mickey to try and understand what happened before the Keyblade War? What if Mickey and Master Yen Sid have known about this all along and have only been acting as spectators because, since it's a prophecy, they can't do much?
+ Young Eraqus/Xehanort in Kingdom Hearts III: I think the opening scenario of the game will be centered on these two. It could solve as an intro to what the plot will focus: on the Foretellers.
These secret endings have given us more questions for KHIII to answer. It's like they're purposely building up the heat for KH3. But if DDD HD were to happen... What more could they add with, yet another, secret ending? A conversation between Master Xehanort and a character we cannot see, but could possibly be the Sixth Foreteller (again, assuming they're separate entities).