The most striking thing about his presence is his age. But he’s not the only one who has undergone an age shift. We have Sora and Riku who also appear younger. Granted, it’s not anywhere near as much of a leap for them, but I would wager that the reason why they’re younger is the same (or similar) reason why Xehanort is younger. In other words, Xehanort appears young in the Realm of Sleep, but he’s really his old self in the real world (as the Re:coded secret ending seems to imply). I’m not really going to speculate on the reason why they can appear younger because, even though Nomura said it’s essential to the story, I honestly think it’s just an excuse to have them in their younger forms again lol. And I can’t think of any reason for why they would need for this to happen.
Although his older self is what's shown in th Re:coded secret ending, MX has fused with Terra, hasn't he? And Yen Sid mentions that Terra's heart is something they still need to search for. That would mean Yen Sid and Mickey don't know about the fusion, right? Though I must admit it's been awhile since I've played BbS, so I'm saying this for clarity.
I also don't know why having younger characters is significant when there various other more important things I can think of, but I'll certainly ponder it and get back to the forum if I come up with anything.
That's a very interesting concept regarding Vanitas' heart residing in Xehanort. It had been bothering me as to where it went, since Ven's heart was still shown as incomplete, so this really clears that up for me. I wonder why Vanitas' heart went to Xehanort's though, because as I see it, hearts don't just go all willy-nilly, latching on to the nearest living thing. But that's a whole 'nother can of worms for another time.Then he would just need the dark half. Vanitas. Now, I know it’s been said, but I absolutely agree with the theory that Vanitas’ heart resides in Xehanort. The trailer itself hints as much.
“Aren’t there hearts confined within you too?”
Of course, YX could be referring to someone other than himself with the “too” but the scene directly after says otherwise. “Hearts other than your own.” With Vanitas magically popping up out of nowhere. It really is screaming, “Xehanort has Vanitas’ heart.”
Alright, we get that. But how did he get Vanitas’ heart? Simple. Vanitas was one of the “seeds” (or “roads” in the English translation) that Xehanort was referring to in the BbS secret ending. He has the yellow eyes which indicate a connection to Xehanort.
This part of the theory is taken from the old Horcrux theory. That MX split up fragments of his heart into various vessels to increase the likelihood of his survival. However, when these beings are defeated, it has an interesting consequence. Rather than fading into darkness or (in the case of some Nobodies) reverting to their original state, these hearts go back to Xehanort. Think of it like a magnet. When their hearts are released from these “seeds,” the bits of Xehanort’s heart within them attract to the primary piece of Xehanort’s heart.
So, in other words, I’m saying Xehanort has quite the amalgamation of hearts now. His own, Vanitas’, Braig’s, Isa’s, and potentially still Terra’s and Eraqus’ (this depends on a few things but that’s an entirely different theory). As YX says, it’s hearts (plural) other than his own.
How this will play out, and what Xehanort intends to do with these hearts, I’m not sure. One possibility I’ve thought of in the case of Isa’s heart is that he may be using it as leverage against Lea. Perhaps Lea has found out that Isa changed due to the influence of Xehanort, and he wants his friend back. With Isa’s heart residing in Xehanort, Xehanort can have Lea do his bidding (notice how he seems to have been tasked with finding Ienzo and Aeleus).
But I also wonder why Xehanort chose Isa as another possible vessel. I know you made a reply somewhere about the darkness within Isa and that's what attracted Xehanort to him (possibly). But why keep him so close when they were Nobodies? Just to use Axel to carry out the dirty work? Braig had better motivation for working with Xehanort (the ability to wield a Keyblade), so why not have him as your second in command? Well, I'm asking more questions than theorizing that, once again, correlate little to your theory. But I like expanding things a bit, considering your theory seems very well thought-out, straight forward, and plausible.