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I think Master Xehanort must be from the past - the present-day Xehanort that was revived after the destruction of Xemnas and Ansem would be Terra/Xehanort.
Master Xehanort, Xemnas, and Ansem must eventually return to the points in previous games when they cease to be - Master Xehanort at the end of BBS so he can become Terra/Xehanort and eventually give rise to Ansem and Xemnas. Then those two need to be returned to their respective moments of defeat, so they can return as the revived Terra/Xehanort. Terra/Xehanort is free to act out his plans without paradox as long as he ensures everyone from the past returns to their proper place.
I think, at some point, Young Xehanort will need to return to his own time, and this will happen before the ultimate plan of Master Xehanort is realized. This way, Xehanort will have a lot of knowledge of future events, but not yet know the ultimate resolution of his plan. This hole in his knowledge may be the one weakness he'll have going into KH3.
There's been no sign of the reborn Terra/Xehanort appearing yet.
He may not even be a member of Organization XIII, standing apart from it as the ultimate enemy. If Xehanort intends for 7 lights and 13 darknesses to clash and form the X-Blade, then he might need to avoid taking part in the clash directly, so he can claim the X-Blade after the war wipes everyone else out.
Kingdom Hearts 3D = Final Fantasy XIII-2?
I saw Young Master Xehanort mention that he would eventually return to his proper time and memory of his actions would fade, but I do not know if this happens at the end of 3DS or if it will happen later. Ditto for the rest of the Org. members.
It seems unlikely, if Xehanort intends to form a team made up of his various incarnations. They'll need to stick around for a while yet. Otherwise he'll need to recruit a lot more team-members, and he implied that the Organization only needed one more to be complete.
i bet that the last member of Xehanort's organization XIII will be the final boss of KH3
So, Ansem is the only one can time-travel?