Honestly I think with Xion it should have been a full on Xehanort possession, because as you stated trying to reprogram her hasn't worked in the past so why would it now? Unless they had someone like Namine rewrite her memories? Though I agree that her being possessed or reprogrammed does little for her character. Though I guess her breaking free of the possession because of her relationship with Axel and Roxas could reaffirm their bond and her strength, but that'd be retreading days' path.I get where you're coming from (part of me kinda wanted to see a Roxasnort), but don't you think nort-ing Xion for the novelty of it would be a bit of... a waste? It doesn't help her character or re affirm her growth in any way, as far as I can tell. It regresses her back into a slave on the premise that she can be "reprogramed", despite the fact that it was already shown you can't program hearts to act any kind of way. DiZ admits at one point that once the subjects of his data Twilight Town grew hearts (through their connection with Roxas), he couldn't get them to "think and act the way he had envisioned." Which is what led him to conclude "a heart is more than any system." This is also why Xion is still able to act of her own freewill and trick Roxas into striking her down (ruining the Organization's plans), despite Xemnas seemingly "reprogramming her" just before she faced her friend in their death match. It didn't work then, so why in the world would it work now?
For her arc to have come to a real, satisfying conclusion, not only would you need to bring up her sacrifice at length, as well as have her re-establish her relationships with her friends, but she also needs to prove her individualality and right to exist as someone just as human as everyone else. That can't be done by making her a slave for Xemnas AGAIN, under the pretext that she can still be "programed" despite developing her own heart and mind. Something Xemnas openly expressed was not apart of their original design, and therefore wasn't something he or Vexen could possibly have accounted for.
Do you see where I'm coming from at least?
Kinda wonder if her Data Battle counterpart will have those armors or echos of them.Again, I just wanted a excuse to see a evil Xion in the game. I just think the idea would be awesome for some reason. Plus her armor forms can come back, lol.