Alrighty, so I've had this idea for a weeks now, and thanks to this trailer, I'm much more confident about it. Since KH3 was first announced, I was one of the ones who believed, if nothing more than for the sake of poetic value (when looking at the title of the game) that there would at least 3 people left to oppose Xehanort when all was lost (KH1- Sora; KH2- Sora and Riku). For months, I personally believed, and hoped, that these potential three would be Sora, Roxas, and Ventus. The opening (from what we've seen) appeared to mildly support this idea: showing the three of them reaching out for the female of their trio before being left alone in the end, indicating to me that they shared a similar goal. Grant it, the openings have always been more conceptual than anything else, so this wasn't the strongest of evidence. However, in the past few weeks, I've since changed my mind on who the final three might end up being.
As odd as it seems, there's more upfront evidence that Terra will be one of the final lights to oppose Xehanort. Bare with me please, but as we've seen from short version of the Big Hero 6 trailer, Terranort is standing at the base of the Rising Ruins. We later see Lingering Will attacking Terranort (much to the later's surprise); indicating to me that LW shows up suddenly and at the last minute. All of this said, my guess is that Terranort will be more of a literal guard for the Seeker's while they carry out the final stages of their plan (explaining why we see 14 gold chess pieces- if we're count Terranort's). This may put him in the perfect position to be saved, and ultimately assist Sora in his hour of need. Putting him as one of the final characters to oppose Xehanort.
Other than Terra, I still believe Roxas will be among the final three for a few reasons. 1) We see him at the base of the rising structure in the BH6 trailer (more on this later); 2) We know thanks to Nomura that Roxas will have a major role, we just don't know WHAT; 3) Roxas saving Sora would perfectly allow things to come full circle since Sora would have just recently revived him. His sudden appearance at the end also makes sense symbolically since we never see his chess piece in any trailer (implying that he could be an unforeseen wrench thrown into Xehanort's plans, similar to Aqua in BBS), and this may shed light on Eraqus' final move after supposedly being in "checkmate."
My resolved for this theory was strengthened by what this trailer shows. It immediately opens up with Sora on his knees pleading for help, all while showing shots of his lost friends. But you wanna know what's interesting? The first two shots are of Roxas and Terra specifically, THEN everyone else. Coincidence? Maybe, until you realize that the same thing happened in the DDD opening. Roxas appeared first, then Terra second; followed by Ventus and Aqua. Furthermore, and correct me if I'm wrong, but have we seen a hint of anyone besides these three (in terms of the guardians) being present when the Seekers are on their rising platform (which I believe is after they already put down the Guardians of Light)? Yes we saw Riku, Mickey, Lea, Kairi, Donald, and Goofy fighting with Sora, but it was more so against a gigantic horde of heartless, Unversed, and Nobodies, rather than the actual Seekers themselves (which may explain how all the guardians were possibly beat while the Seekers seemed t remain perfectly fine [something I always found strange]). And we haven't seen even one shot of Ventus or Aqua in what I'm assuming is the end game scenario (suggesting that, for whatever reason, they may end up sitting this one out).
Overall, I somewhat like this idea. It gives Terra the chance to properly make up for his mistakes (fulfilling his promise), and it gives Roxas the chance to truly stop being selfish (unless forced otherwise) and finally repay Sora for working so hard to save him (something he himself was beginning to doubt was possible in DDD). I imagine there will be some heartfelt words between two (Roxas & Sora). During it all, Sora may finally end up fulfilling his destiny. Also- and I just now thought of this, if Roxas is the one to pick Sora back up from his despair (which would explain the upward shot of him in the BH6 trailer, since Sora's on his knees), then he'd be the "key" to helping Sora unlock his destiny. Sort of explains why he was thematically given the title: "Key of Destiny," doesn't it?