Well, I already said my theory that it might somehow be
Spoiler ShowApprentice Xehanort from the future
But I'm not sure. XD
No, the LAST thing KH needs is to start bringing in time travel.
Here's my theory on this secret boss: he's wearing the robe that we all thought Master Xehanort wore first, right? Well, I think HE wore it first because HE is Xehanort's old master. It makes sense. And it can't be an organization member since they haven't formed that yet.