So theres a lot of speculation on this, and my first thought was rapture rising from the sea, but i think ive got it now.
The girl is one of the little sisters, now in her teens, that jack saved from rapture in the good ending. only instead of living normal lives like the others she went all "carrie" and has similar plasmid abilities to jack and the splicers. maybe jack made the big daddy doll to give her something familiar to attach to when she was little and now its bringing back these long blacked out memories or rapture and she is unconsciously shaping the sand behind her to the images in her head as she looks out at the sea.
Perhaps it doesn't even take place on rapture, and this is just the catalyst for it, im definitley getting a carrie/ring vibe of the girl, but only time will tell. aside from a terrible endgame boss bioshock was almost perfect, maybe it could have used some more period music but the art deco style just captured my imagination. So needless to say i cant wait for this.
But seriously folks, the trailer, whats your speculation about it?