“Diablo II, to me, was a great model for an expansion, because it enhanced the original game, but also extended the game, too,” Levine told Games for Windows Magazine, according to 1Up. “I'm not a really big fan of expanding things just by linearly adding to the experience, adding a new campaign, as much as I am of enhancing the original experience and adding replayability to that experience.
“I think that certainly BioShock's combat experience is great, but it could be broader. I'm a little more confused as far as how to expand the narrative experience.”
Apparently lots of plasmids, the in-game super-powers afforded to players, were cut during development – a likely route for the team to follow.