I always figured that voice was Mickey. As for Hollow Bastion containing the door to Kingdom Hearts, Ansem the Wise asked himself the same question in his final report in KH2, so I doubt they've forgotten about that. It will probably be something left for the prequel/spin-off/sequel/whatever in God's name they're making next. And didn't End of the World only exist because most of the worlds had been destroyed by the Heartless?
I'd really like to know for certain when this trailer takes place before coming up with any theories, but that's not going to stop me anyway.
Assuming that the trailer takes place in the future, one possible theory is that the darkness that was in Riku's heart (Xehanort's Heartless) that had left him when Kingdom Hearts exploded (y'know, when he turned back to normal) managed to find its way to Ansem the Wise's body and left before anyone woke up. And don't say it's not possible, to quote Mickey "Ansem did say anything can happen." Considering Xehanort's Heartless wanted to be Ansem so badly, this really isn't that far-fetched.
If it takes place in the past, and that really is Ansem the Wise, it would explain why he was hell-bent on preventing Xehanort from conducting research on darkness, the heart, etc. and why he flipped on him in front of Mickey during the flashback. He never said why he was so strongly against it, not even in his reports.
Yen Sid's appearance was just a cameo, nothing more. That's a stupid theory, sorry Xiron.
Edit: As for the grown man with brown hair, his eyes turning yellow doesn't mean that he becomes Xehanort -- besides, Xehanort's eyes have more of an orange tint to them. If anything, that along with the fact that his skin is slowly turning black seems to indicate that he's turning into a Heartless.
Not only is Sora using it in the video in which he fights the Knight (and dies in three seconds), but Xehanort's Heartless(?) is using it in the secret vid. What's the story significance supposed to be with that thing?
