If this theory turns out to be true then it would be possible for MX to have I guess transferred himself into Terra, maybe he wanted Terra to give in to darkness so that it would be easier for him to take control of Terra, this would also explain why Xemnas referred to Aqua's armor as friend. It could be the part thats still Terra manifesting through little bits of memory sort of like the way Sora recognized Hollow Bastion/Radiant Garden even though he had never been there before.
I'll break this down, because i had a theory for this as well,
but i'll help adjust the theory you have given, because it
would actually destroy my theory if this were true. We don't
want to place so much into ppl transferring with ppl and so on.
Now if you are willing to look into spoilers, by all means read,
but be aware that it is a very essential part of 358/2 days so
you have a choice.
Spoiler ShowXion was made by Xemnas through the leaked
memories of Sora. The strongest memory of Sora was Kairi, so
some ppl saw her as a Kairi look-a-like. Others saw her as a
hooded figure, and Braig and sometimes Xemnas, witnessed her
Now we can allow ourselves to believe a similar situation with DS
if things supposedly do come close to my theory. If the heart of
MX, lies within DS, then so do those memories. The heart will give
shape to his empty shell and just as Xemnas created Xion out of
Sora's memories, The DS reflects from the memories of MX.
The only thing that hinders those memories is the fact that (if DS
really is the Xehanort we know) he lost them. In certain aspects
he tries to regain those memories. One memory being presented very
clearly is the clothes in which his heartless form wore being similar
The Difference between this theory and how Xion was created, is
that the memories come form a heart that is already placed within the
DS. If those memories are awakened we may actually come to see
MX once again. (I would randomly guess that the heart was sealed,
but by who is the question, and the mystery).
It all boils down to Xehanorts memories, and him trying to find them.
If Xemnas regains memories through the room of Sleep, as Sora's
memories were linked together in the lotus pod, then all that's left is to
retrieve a heart, not of the one he had when he was Xehanort, and use
the room of awakening to awaken those memories within that heart,
then he wouldn't become MX, but rather his own being, and with that
existence he would gain control of his artificial KH(that is if Sora didn't
hastily foil his plans) If however he was Xehanort and used the same
process, then MX would return awaken from the heart left within Xehanort.
Xehanort will surely return, seeing as his heartless was destroyed before
his nobody, and if the memories of the two entities combine within
Xehanort, then surely he will know how to regain his memories, and he'll
surely know where to return to KH as well. If this theory does come to
fruition, then this will considerably be a major coincidence.