Watched the trailer to BioShock 2: Sea of Dreams yet? You should. It certainly gave us a lot to speculate on. Now speculate on this: a developer from 2K revealed that BioShock 2 is a prequel, but at the same time is a sequel. Think, think, think. Here's the quote from 2K Marin Developer Charles Shechbeck:

BioShock 2: Sea of Dreams is part of a prequel and at the same time is a sequel, so you’re kind of close on the interpretation of the trailer. The rising replica of Rapture can represent something to do with this or something part of a bigger surprise that everyone has to wait and see to play.

So we know that the sequel occurs after the events of the first BioShock, since we see a Little Sister has hit puberty. I assume it's a Little Sister since she has a little Big Daddy doll (LittleBigDaddy?). Besides, who else could it be? Then we see the skyline of Rapture rising out of the sands, so we know the city is still a part of the story.

Thoughts? Here's the trailer again for you, see if it gives you any ideas: