People are too swift to assume that what we believed prior to BbS is right. Xehanort's Heartless is dead, Xemnas is dead, so, in accordance with what Nomura said in Another Report, Xehanort (the apprentice)
must be back.
But considering that we now know that Xehanort has the combination of two (or three) hearts within him, the answer may not be so simple.
Let's take it one step at a time, starting with some basic assumptions:
- When Xehanort the apprentice willingly released his heart, this heart was a combination of Terra, MX, and Eraqus' hearts. With MX being the predominantly dark part of this heart, the heartless (Xehanort's Heartless) was primarily under the control of his sense of self. Meanwhile, the leftover body and soul, which were essentially at one point Terra's, becomes Xehanort. Terra's sense of self is primarily in control here, but he's still affected by Xehanort's darkness.
- Since Xemnas was alive and well when Xehanort's Heartless was defeated, the heart from Xehanort's Heartless would have normally gone into a "state of suspension" (as Nomura calls it). But it instead lingered in Riku. In other words, Riku contains, essentially, MX, Terra, and Eraqus' hearts.
So, in order to get
some kind of Xehanort (whoever it may be) to return, at the very least, both his Heartless and his Nobody must be defeated. We know Xemnas was defeated.
But what about Xehanort's Heartless? As I said, even though he was defeated, the heart still rests within Riku, unusable by Xemnas' body and soul. Well, not all of it anyway, thanks to AtW's
explosion.
By answering what happened to Xehanort's heart in that explosion, we'll know who the next Xehanort will be.
Now, how did it effect Riku? It reverted him to his original self and, as Mickey said, made him no longer part of the dark realm. While I'm sure he's still capable of using darkness to a degree, I believe this implies that he was finally rid of the influence of the darkness of Xehanort's heart.
But just the darkness. In other words, the "MX" part of the heart was kicked out. But the Terra and Eraqus parts of the heart still rest within Riku.
So now we know that, when Xemnas is destroyed, the body and soul can combine only with Master Xehanort's heart. Now, while the body and soul may technically belong originally to Terra, the presence of a heart is what affects a person's appearance. This is why Xehanort looked like a combination between Terra and MX, and why Roxas (with only Ven's heart in Sora's body and soul) looked like Ven.
Thus, with only MX's heart, this being should, obviously, take on the appearance of MX...
except it's a younger body. Therefore, it should look like a younger MX. Hence, what we see in that picture is what we'll get for the "next" Xehanort.