I've spent days analyzing this trailer, and this is what I've discovered:

1. As we can all see, the Little Sister is not-so-little anymore. While many would go for the "All Grown Up" explanation, notice what she's holding in her hand; what appears to be a Big Daddy doll. Or is it a doll? The things sprouting from the ground around her are clearly meant to be or at least symbolize buildings. I suspect the sequel will deal with genetic experimentation gone wrong as the experiment code-named Big Sister grows out of control and wreaks havok on the already havok-wreaked Rapture. Then again, she is clearly on land, which could signify that the city she rampages through and destroys is a dry city. Judging by the Art Deco aesthetic and upbeat coastal imagery, I would bet if this is the case, the city will be none other than New York.

2. The Big Daddy doll, assuming it is a doll (see point 1): Is this some new item in the game? My theory is that it is used to solve puzzles and fight enemies, and is upgradeable. Also, the plasmid you currently have equipped changes its properties. No doubt it will be a powerful ally in your fight against...

3. ...the Barnacles seen at the end of the trailer, growing on the logo. I suspect these are no mere aesthetic detail. A question which never came up in the game is whether these genetic experiments extended to extrahuman subjects. Barnacles, unsightly yet innocuous creatures, could have been a target of study...perhaps as next-generation living oxygen scrubbers? This seems likely, given the oxidation of the logo coinciding with the barnacles' appearance.

4. That ostentatious butterfly. What's it doing there? I've checked and cross-checked, and narrowed it down: it is none other than the Menelaus Blue Morpho butterfly of Central/South America. Often identified with Aphrodite, the Greek goddess of love, the butterfly walking around the logo depicting the city may very well symbolize a female player-character making her way through this new adventure. If this does not happen I will be very surprised. This theory is further reinforced by the background song's female vocalist.