
Ken Levine is hoping to extend your visit to BioShock's Rapture by introducing some downloadable content in the months to come. Just don't expect any more storyline.
Speaking to 1up, Levine explained that his vision for BioShock DLC doesn't include new levels or expanded plot so much as it centers on improving the existing experience:
Apparently, a number of plasmids got cut during the game's development, so perhaps the DLC would simply add to the roster of tonics, plasmids, or ammo types. Any reason to revisit Rapture is a good one, as far as I'm concerned.