Seeing how there was a flashback scene of Xehanort attacking Xigbar, Aeleus and Ienzo in one of the trailers for Kingdom Hearts 3D: Dream Drop Distance, I think it's safe to assume that Birth by Sleep v2 are just compiled scenes that will be featured in Kingdom Hearts 3D: Dream Drop Distance.
Why would they refer to something from 3D as "Birth by Sleep volume 2," though? Using that sort of title suggests a deeper sort of connection to BbS than 3D has demonstrated so far (if anything, it seems to be more interested in the Organization than TAV).
I mean... there's a lot of stuff in that teaser that seems unlikely to show up in 3D, from what we know of it. Right from the beginning, there's the Castle of Dreams showing up in the Realm of Darkness, which would be hard to explain in a game focused around the Realm of Sleep. There's the line, "A Fragmentary Passage," which doesn't really seem to connect to 3D in any way I can see. There's the scene with Kairi at the mansion in Twilight Town, which makes the most sense as a flashback but could never appear as such in 3D due to the fact that Kairi's not even in the same realm as Sora and Riku are. There's the different perspective on Donald and Goofy before they run into Sora, which would be just as hard to justify.
And, even if the rest of it could be worked in as Riku flashbacks, the tone of it is completely different. If they were teasing 3D, there's no way Sora wouldn't have been more important -- every single scene, apart from a single shot of him sitting on a tree trunk before disappearing, focuses around stuff that happened when he wasn't there. 3D is, most definitely, at least half Sora's story. There's no way they would leave him out of a teaser for it.
Everything in that teaser points to it referring to a game without much Sora involvement, with some sort of fragmented story. It opened up a new option to use the Realm of Darkness as a setting by sticking destroyed worlds there (because, let's be honest, it would have been a terrible setting for anything more than a Secret Episode otherwise), at the cost of adding even more questions that need answering, and 3D can't even take advantage of that because it takes place too late in the timeline for those worlds to even be there anymore.
To be fair, the idea of a fragmented story is... kind of weird, but I could picture some kind of frame device being used that could somehow tie in a whole bunch of disparate memories and characters together, if they wanted to go that route.
