Crispy Gamer takes the opportunity offered by the BioShock film to editorialize on what they think might or might not work.
BioShock as a game has one actor for most of the story. A single survivor of a plane crash is given missions and advice from a mysterious interlocutor. You control a guy with guns and lightning bolts getting instructions over a radio. The explanation of what he is seeing comes entirely through recorded diary entries and the occasional really big enemy to kill. There are no sane one-on-one encounters until the big turning point.
Can you establish the cast of BioShock and still keep it as a game about shooting things and setting them on fire without increasing the number of people doing the gunplay and arson? This would take it down the path of the Doom movie, which was not about one super-marine, but a squad of them. BioShock, though, has a moment that is, essentially, about individual choice. Will that be as powerful if you have an entire planeload of people running around Rapture?
Then you have the iconic figures of the game -- Little Sisters and Big Daddies. The choice of whether or not to harvest ADAM from the Little Sisters can only really handled in film by a dialogue. Internal soliloquies don't work quite as well, I fear, but that means adding a second person at least. In the game, Big Daddies are a constant fear, but can't simply become a movie monster and maintain their mythological power. You only have two hours in a movie (or maybe three if the last two bloated Pirates of the Caribbean movies are any indication) so you have to decide when and why you deploy your Little Sisters and Big Daddies. The protective relationship between the two is central to its creepiness, but it wouldn't take much for a screenwriter to warp that into an exploitative relationship that adds further motivation for the protagonist.
Because the setting doesn't necessarily have to be all about turning corners and killing things, there is also a greater likelihood of introducing a romantic subplot. Not everyone in Rapture needs to submit to the madness, after all.