To your question, though, I'd definitely love to see an original story set in Rapture. Perhaps from the perspective/origin of the splicers/Big Daddies/Little Sisters in Rapture combined with some narrative focus about what was going on in the world above that was the catalyst which made Andrew Ryan conceive, fit the bill and ultimately build Rapture in the first place. Sure, some of that is explained in the games, I just think it should be a movie which you can enjoy without having played the games. Some easter eggs for gamers would be super important, too. It's a rich and intriguing mythos that would definitely make for a good film imho.