Ok, I literally
just registered only so I could explain my own thoughts.. so here it is(sorry if someone has already word for word said this)-
I don't think Riku would willingly turn on his friends. Think about what you're saying- a year ago Riku was a completely different kid; he was extremely competitive with Sora, he was jealous of him when he made new friends(and to an extent he was jealous of Sora's love for Kairi, after all he did have sort of a crush on her back then). Riku always had a strong heart but he let his negative emotions affect his heart and it eventually bred darkness inside him, which was expanded upon when Maleficent corrupted him and Xehanort possessed him.
Point being, Riku has significantly matured since that time, at the end of KH1 the guy was willing to stay in the realm of darkness just so the worlds(and his friends) could be at peace again, and even went as far as trusting Kairi's well being to Sora. Not to mention his time in Castle Oblivion and what it taught him. He realized he would have to defeat the darkness in his heart(eventually he learned that he could use it as well as he could use light, but he specifically had to defeat Ansem), he was overconfident and arrogant in KH1 but now he knows it's okay to ask for and accept help if need be. When he heard that Sora was in the castle he was surprised and happy, but when he found his friend asleep, instead of being too sad, he resolved to put an end to Ansem and later came back to watch over Sora. To reiterate things already said, he went over and beyond for Sora, he even went as far as to accept the darkness inside him and took on Ansem's appearance
JUST so he could defeat Roxas and let Sora wake up. Not to mention the fact that even after Sora woke up, he
still looked after him and helped him along the way, and Kairi too. When they're all reunited, he explains many times over that he didn't want to be found. His friends were out there in the worlds looking for him and even if he wanted to see them, he gave up his own happiness because of his shame.
I find it
extremely hard for this boy to willingly go back to the man who has tried ruin his life and his friends lives, for the same reasons as he did before.
Now for my thoughts:
I think Sora
and Riku will pass the Mark of Mastery, they have both been shown to have strong hearts, and Riku has proved to be able to conquer his own darkness. Now for the iffy part of my theory.
If Ansem and Xemnas are destroyed Xehanort will come back. Yen Sid himself said that more than one form of Xehanort will emerge, which means that most likely we are looking at Ansem, Xemnas, the 'Original' Xehanort, and Master Xehanort coming back. That's the real Xehanort(MX), Terranort(Terra's body, MX's heart), Ansem(the heartless of Xehanort's heart), and Xemnas(the nobody made from Terra's body).
Terra's heart never left his body, it was only taken over by MX's heart. Which means that if his heart is still in Terranort, than he can't use his original body(if he did and his heart left Terranort, then Terra would be able to come back-atleast that's the basis of my theory). So basically, his old MX body is like a backup if he looses his younger host-
whoever that may be at the time. Right now it's Terra, but Riku has shown to be as strong or stronger than Terra, as young as he is. However, age is not an issue for MX because the younger his host the more longevity. Strength matters most to MX, he was going to leave Ventus for dead when he saw that he wasn't strong enough to be his host, and pursued Terra instead. I think it's safe to say there is some kind of connection between MX and Riku- a few things give life to that theory:
The vision that Terra has of young MX, then an older Riku, which occurred after Riku had been talking about Xehanort(though, he didn't know him personally), meaning he had been taught stories about him.
In the Blank Points when MX and Terra are having a conversation in Terranort's mind, MX tells Terra that there were
'other roads he might choose to take'. Now what could that possibly mean besides possession? Also notice how up to that point Terra was calm but is now angry, while MX is very confident after this.
Finally, in KH1 Ansem approaches Riku after he had lost the Kingdom Key to Sora. He
sought Riku out. He couldn't have just randomly picked Riku as a host. Also just a side-thought, why would Ansem want to possess Riku anyways?? Really? I'm confused as to what purpose that served.. what could he do in Riku's body that he couldn't do in his own? IDK maybe I missed something, or have forgotten something that happened.
So here is my final thought:(didn't think I would ever get to it
)
Riku and Sora are brought up to date about everything that happened back in the BBS days, then their memories[the ones that are sleeping in their hearts(reference to Coded, what Namine tells Sora and Mickey)] come back to them and they remember TAV, and Riku remembers that Terra is his master. So he thinks he might be the only one who can save Terra(assuming at this point they were close to saving or already saved Ven, Aqua, or both) so he lets MX take over his body, meaning MX's heart has to leave Terranort, freeing Terra(and then all they have to do is connect his heart and body with his spirit and will-Lingering Will in Keyblade Graveyard). So Riku sacrifices himself to save Terra and(insert fuzzy part of theory) either overcomes MX and forces him to take back his old body(that would depend on how strong Riku is at this point) or he trusts his friends to save him.
Of course, this is only if Riku has anything to do with 'shocking ending'. If not then....I don't know what's gonna happen.
Well, that's all. If you don't agree with me that's fine, just please don't flame. It's my first time telling anyone else what I'm thinking sooooo...yeah.
Edit: huh just saw I'm the only girl that's posted on this in a little while.. all the rest of you are guys lol