Alright...here is my guess from looking at the teaser trailer and reading the lyrics of the song used in the trailer.

I think that the girl was a little sister, but was saved and taken back to normal society, however, she never forgot of Rapture. She remembered the Big Daddy's protecting her and remembered the Little Sisters. She begins to feel lonely, so she goes back down to Rapture, trying to return Rapture with Little Sisters and keep the balance going. From reading the original website for the game, I believe she becomes the Big Sister who is kidnapping regular girls and turning them into Little Sisters to feel the gap of loneliness.

Now...as for reading into the trailer and what exactly the girl is doing. She's thinking or dreaming of Rapture, that much is obvious. The first line of the lyrics are Dream
When you're feelin' blue, therefore, she is upset from being taken from Rapture and so she dreams or thinks of Rapture. She has the telekinesis ability, which I read in gameinformer so she is building the city of sand, and you could have also of guessed this from her awkward twitching of her head. Another line of lyrics...You'll find your share of memories there
So dream
When the day is through, so this is a sunset, not a sunrise. The day is through, she is thinking of rapture. Last line Dream
And they might come true
Things never are as bad as they seem, so she is dreaming of having Rapture back to it's Big Daddy and Little Sister ways, and she wants this so badly, that they'll come true. And for your info, you play as a Big Daddy in the game, I think that this girl is a Big Sister. Which were just introduced to the newly made Bioshock 2.