Kadaj seems to refer to a "mother," which is obviously suspicious, especially given his resemblance. The obvious assumption is that it's Jenova he speaks of, which correlates to him referring to Cloud as a brother, but one problem with this is that it seems to imply that Jenova is still alive. While this is a common theory (and one that I believed up till now, actually, when I started to think about it. xD), the entire cause of the Reunion was because Jenova's cells pull towards her. So wouldn't Cloud know if she was still alive? Wouldn't he have tried to destroy her for real? Of course, if she IS still alive, that only continues to prove the idea that she might have been the one in control all along.
Since the Turks are dealing with the WCM, Cloud speaks to him, and he seems to speak of Sephiroth in third person, I think it can be assumed that he is not Jenova's son. (The only problem with this idea is how much reverence Kadaj seemed to have in some of the earlier trailers, though I'm not certain) Either way, we know that he's in the Healin Lodge, as some people were speculating.
Aeris is somehow reachable by Cloud, though she isn't alive? This bit confused me. Like, if he can talk to her, can he do the same for Zacks, or anyone else?
There's a lot of things it ties up, though - the scenes we've just seen brief flashes from and pictures from make sense now - you can kind of imagine the order that they're in now.
And one thing I found really, really odd - most of the assumption that Geostigma/SSS was caused by Jenova was due to the idea that Cloud DIDN'T have it, but I think with this trailer we have to discount that, since he definately does. But it also seems to imply that Jenova can cure it, and that she caused it. Interesting enough, since if anyone will care to recall exactly how she killed most of the Cetra - a virus (presumably spread to people through the lifestream, same as SSS, though I might be pulling stuff from my butt. It's happened before.) Still, if it's caused by Jenova's poison, it would seem like a clone of hers would be immune to it, even a failed one? This also opens up the question of what those who have faulty reactions to her cells react like - would it be something like Geostigma? It would certainly seem to imply as much.
So, that was just what I got from it, would anyone else like to discuss theories or spoilers or anything? =) (And does anyone know if there are any pictures from this available? =o~)