Here's a compilation of some small theories I have. Obviously the amount of proof I have for these are limited due to it being centered on BBS and characters from BBS.
Now, I know for these first theories, someone who knows nothing about the KH plot will yell "Well how come their Nobodies didn't wield a Keyblade?" First off, one of the main qualifications to have a Keyblade is to have a strong heart. Nobodies don't have hearts, so how could they wield one? "But what about Roxas and Xion?" Xion's was a fake, although a nice imitation. As for Roxas, have you played KH2FM or Days? Clearly you didn't if you're asking such questions. In fact, let me just say it.
Those are the main theories I had, but I also have one or two smaller ones, which some moron might scream "old news" to.
That's all I have, for now. If you like the theories, that's nice. If you disagree with my theories, that's nice. If you disagree with my theories, yet won't post any decent reasons or opinions why, then why are you posting?
Enjoy.
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Theory #1 - Terra is connected to Xemnas/Xehanort or Saix. However, as of lately, I feel especially strongly towards Saix's connection even though I believe Xemnas' would make more sense.
First off, we can see that their hairstyles are similar, only with Saix's being longer and blue. The fact that they have different haircolors hurts me, but I still don't want to ignore the hairstyle and overall posture. Another thing that connects them is how they react when the Kingdom Hearts moon is involved. We all know how Saix reacts, he goes berserk. At the end of the KH2FM trailer, we can easily see Terra is full of rage, and perhaps about to go mad as well. Even his eye colors change to the same color Saix has. For all I know, another sideeffect of the moon could have changed his haircolor too. Although, what hurts this theory is Saix's name. It's not an anagram of Terra X, although Xemnas proved that they don't need to use an anagram of their real name, since we all know Xemnas isn't Ansem, but Xehanort. For all we know, Saix could just me an anagram of an alias. Wouldn't it be interesting if Terra was connected to Saix, and Xemnas were connected to Master Xehanort? Their whole forms as well as their bodies would always be in conflict, as we know thanks to 358/2 Days that Saix wasn't as loyal as we previously thought. Terra had an interest in finding (more about) Master Xehanort, and Saix had his questions about Xemnas. Can this be pure coincidense?
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Theory #1.5 - I believe this one mainly if Theory #1 is true. I'll be blunt, Vanitas is Lea. I'm not holding this theory as strongly has Theory #1, but it's still one.
Instantly, we can all think of two things wrong with this. Firstly, Vanitas is in no way the same name as "Lea". Unlike Saix's situation, we are positive that Axel's somebodies name is Lea. Another problem is their body. Vanitas looks a lot more built than Axel does.
As for something else, while it may not directly support this theory, it certainly doesn't harm it. Why does Vanitas wear that mask? The fact that they hid his face all this time must mean it has some significance. I seriously doubt after all of the mystery surrounding this character, they'll finally at the climax of the game have his mask removed only to reveal that his face is of no significance, he looks like no one we've seen before, and he's a character completely independanty of everyone else.
The reason for thinking this is we know Saix and Axel were connected, possibly friends, as their original selves. If Terra is indeed connected to Saix, then I wouldn't be surprized if he had some form of a relationship with Vanitas. Afterall, we know nothing of Vanitas' connections with Terra, so it could be good, bad, or nothing for all we know.
The reason I say this is because I'm uncertain if Lea would appear in BBS, assuming he's not Vanitas. However, due to the fact that Days said Saix and Axel were connected, probably friends, in their past selves, I would assume we would see some of that in a future game. If not, then why go through the trouble of putting that in the game? It seems fairly useless to just leave alone.
Now, I know for these first theories, someone who knows nothing about the KH plot will yell "Well how come their Nobodies didn't wield a Keyblade?" First off, one of the main qualifications to have a Keyblade is to have a strong heart. Nobodies don't have hearts, so how could they wield one? "But what about Roxas and Xion?" Xion's was a fake, although a nice imitation. As for Roxas, have you played KH2FM or Days? Clearly you didn't if you're asking such questions. In fact, let me just say it.
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Theory #2 - Roxas has a heart. Whose heart? I dunno.
Sora's? Maybe part of this or something?
Ven's? Terra asked Master Xehanort what he did to "his" heart, probably Ven's. Maybe he did something to it that caused Roxas to have it later on?
His own? I don't know, that's not what this theory is. I'm just saying he, in some way or form, has a heart. It explains perfectly why he can use the Keyblade and people such as Xemnas or Saix wouldn't, assuming at least one of them is connected to a Keyblade wielder. Also, the fact that Roxas having a heart has been hinted at many times throughout the series doesn't make it unbelieveable. Even Saix theorized in one of the secret reports that it might be due to his young age, but I can't say for certain if that's true or not.
Those are the main theories I had, but I also have one or two smaller ones, which some moron might scream "old news" to.
Spoiler Show
Theory #0.1 - Master Xehanort says something like "That person is a monster infused with darkness, please correct my mistake" or something like that. Most people assumed he was talking about Vanitas, which isn't a bad assumption. But why would he say something so bad about someone who appears to be his ally? Besides, we all know near the end of the game he's still teamed up with Vanitas, which obviously takes place after he said that. But the question is, why would he say that and then continue teaming up with him?
Because I don't believe he was talking about Vanitas, but about Terra. I would think most people assumed he was talking to Terra, but I believe he was talking to Ven or Mickey. We all know Ven was being knocked down by Vanitas at that place and Mickey came to save him. Perhaps Master Xehanort appeared after, or even before that. Afterall, we can't see whether anyone else is there or not.
But why Terra? We can see throughout the trailer that Master Xehanort is telling Terra to control the darkness and whatnot, and seems very interested in it. However, for some reason, it's possible he regretted that later on and told Ven/Mickey to "correct his mistake." He even says those who submit to darkness are not qualified to wield the keyblade. Which I find interested because one would assume Master Xehanort uses darkness himself... unless he uses it but doesn't submit to it? I dunno, that's not what this theory's about.
Theory #0.2 - "Erase me." I don't think it means death, I doubt it would be that blatant. I think it has an alternate or deeper meaning, which might be his connection to Roxas. An unclear interview stated coded has deep connections to Days or coded. It was implied coded, but some people said due to the weird wording, it might have been Days. I'm unsure myself, since we do see Days has connections to BBS, but there seem to be very few, or not enough to be called "deep connections". However, ever notice how weird they summon their Keyblades in BBS? It almost seems like they're summoning data. I wonder if there is a deeper meaning behind that?
That's all I have, for now. If you like the theories, that's nice. If you disagree with my theories, that's nice. If you disagree with my theories, yet won't post any decent reasons or opinions why, then why are you posting?
Enjoy.