I've been thinking about this recently...
More with Xehanort.
Now, let's say that Xemnas and Ansem SoD are Apprentice Xehanort's nobody and heartless respectively.
Ansem SoD, being the heartless, possessed the heart of AX, while Xemnas, the nobody of AX, was in possession of his leftover body.
Now, when a heartless is defeated and the heart released, and the corresponding nobody is also defeated, releasing the captured body, they both come back to the whole.
This is where my question lies. If Xemnas grew his own heart, where would the heart of AX go once Ansem SoD was defeated?
Xemnas, the body of AX, now is already occupied by a freshly grown heart.
So where would the heart go?
I honestly cannot even begin to think where to answer this. Back to Master Xehanort? That's how he was able to continue to make more copies of himself?
Think about how many times he had to release his heart into someone else. Surely he would of created a heartless and nobody each time?
And after each iteration, he would have most likely grown his heart back, but still wouldn't he be without a body?
Unless, of course, the new Xehanorts he created then gave up their own hearts, thus forming some kind of domino effect. For example, Xemnas.
In the case of Saix, it really looks as if Saix didn't willingly want to become a Xehanort. Which is why he has the recusant's sigil carved into his face. Someone must of forced it on him. Namely Xemnas. Once he grew his heart, perhaps it was he that performed the heart transfer ritual.
If you look at Saix and Xemnas' eyes they are pretty similar, and Saix and Xemnas both have a similar build, and similar hairstyle(please look up images for a comparison).
It could be that instead of turning into Xehanort, Saix is turning into Xemnas(who is also Xehanort in a way).
A bit off topic near the end, but still, i really hope all this is all explained in KH3.
