Just a couple of thoughts I've been having that I'd like recorded somewhere.
First off, your theory and compilation here is amazing.
Little theorylings I have... I believe Ven and Roxas are somehow one and the same. I'm not saying Ven is Sora's Nobody, though. I'm not even saying Roxas is Sora's Nobody. I believe everyone believed Roxas was Sora's Nobody because that was the fact that was presented to everyone... the fans of the game, the people of the game, the characters within the game, even Sora himself.
If you recall, Xemnas gave Roxas the name Roxas. He came from Sora, and thus Xemnas gave him the name Roxas to signify that, however Roxas' true name is (in my belief) Ven. Sora held Ven's heart within his own, and when he released Kairi's heart he also released Ven's heart. Granted, this makes things very complicated because then Sora held three hearts within him; his own, Ven's, and Kairi's. Or perhaps Ven's heart was really Sora's heart, and Sora regained a semblance of his own heart when Kairi willed him back to reality. Now I'm just confusing myself, but bear with me.
I'm also going with the belief that Xemnas, Xehanort, and Terra are somehow one and the same, at least in some sense. Going on the Terra fell to darkness and allowed Xehanort to take control of his body, anyway. However, since Xemnas truly is Xehanort's Nobody, he didn't have the heart to realizel that Roxas is actually Ven, thus being why he simply used him for his own means.
Also, another thing to note is that... someone (maybe OP?) suggested that Ven (or his armour, at least) might be in the Room of Awakening. Well, where do Sora and Roxas obtain their Keyblades (well, when do we choose the path the Keyblade will have... strength, defense, or magic). The Station of Awakening. Sora, Roxas, and Ven are obviously connected and are somehow all one and the same in some sense.
There's also a saying that history is doomed to repeat itself. From the looks of the secret video and all of that sort of thing, Terra, Ven, and Aqua were three friends... three knights of some sort... perhaps just really powerful weapon wielders who were wearing armour for an impending doom... either way, they were close, just like Riku, Sora, and Kairi were. Riku and Terra mirror each other as the protective fighter type, Kairi and Aqua mirror each other as the strong willed type, and Sora and Ven mirror each other as the slightly reckless type. I think that whatever happened during the secret ending (which Mickey witnessed) is doomed to reoccur, only it will most likely have a different outcome in the end. When this will happen, we won't know, but I'm certain we won't find out for some time.
I believe the next installment in this storyline is a sidestory (possibly taking two games to discuss) not starring Sora and Co., but most likely featuring them at least through mention or eludement. Yes, that saddens me, but I believe the current story of Kingdom Hearts (Sora's story) is far from over. I truly expect Kingdom Hearts to go beyond KH3... I expect to see Kingdom Hearts Five in the distant future still dealing with Sora's Story. Too far beyond that might be a bit of a stretch, but knowing the team behind the game series they'll make it believable and desireable to continue the story if there is one to continue. I mean, we thought Roxas was over and done with, but Nomura obviously wants to talk more about him (obviously due to Roxas' popularity with audiences, but Nomura is smart in that he's giving the crowd something they want). So, I don't think Sora is far from being over and done with. There's no way in heck he's settling down to live out the rest of his life without care on Destiny Islands... he's got a destiny to fulfill yet
And he's also got a history's future to rewrite.
EDIT: Oh, and one thing... I hate any part of any theory that suggests Sora is going to either die, or poof somehow, or "become one with Kingdom Hearts" lol... not that I don't love a good theory... I just like Sora as a character too much, and I want to see him develop more. I believe once the series is said and done that Sora more than deserves to finally have his life back and end up as a normal kid (or adult, if it goes on long enough) again. He deserves to have peace in the end... he doesn't deserve to be vanquished, and he most certainly does not deserve to become a martyr and "become one with Kingdom Hearts" or something. The last idea is too cliche and too heartbreaking, especially for the die-hard Sora fans (yeah, you're out there. Admit it. Otherwise you wouldn't be world-hopping in bondage cargo shorts and giant sneakers). So, here's hoping that when Sora's Story does come to an end in the future, that it's an ending that will have Sora come to a happy finality that will leave fans thinking "yeah, that was awesome. Glad that's how his life turns out, because he deserves that happiness."