Firstly, who is doing it? A director with a lot of power like Spielberg or Peter Jackson could get a big budget and be allowed to imprint a vision on it. Plenty of other directors may be talented, but ultimately do not have much power and would have to work the film to a nice, safe, cliched Hollywood formula to appease a studio if it were big budget.
Spielberg would probably like Bioshock as it involves a sort of redemptive family plot - he loves that sort of thing. On the other hand, Spielberg has a way of making movies a bit anodyne - I doubt he could really express the horrific downfall and degradation of Rapture. Peter Jackson could.
Anyway, if it ended up with a director who didn't have the power to impose a vision on it, I fear you'd get a very generic action movie, substantial plot alterations (they'd have to include some sort of strong female or love interest, surely), and the Objectivist angle probably erased or dumbed down to transparency because it's a bit too much to ask film goers to suck up a bit of philosophy with their popcorn muncher.
