I think Geoffrey Rush would make a great Andrew Ryan. He did a great Vincent Price in The House on Haunted Hill remake. And Andrew Ryan is very much like Vincent Price.
This is the first time I believe I've seen this crowd excited about a video game/movie translation. If this guy were directing a Half Life movie, would it get the same positive response?
I'm also with riomhaire on the opening scene - movies (like books) tend to have success with a 'flashy' opening scene to hook the audience, and the New Years Ball (which'll be weird in and unto itself as the first look at Rapture) being raided and then perhaps alluding to the fall and ensuing decay of the entire city would be a good way to go... the audience will be curious about what state this already bizarre city will be in when the protagonist arrives later.
I have a really good feeling about the bioshock movie , i think it would be a good move to follow a group (hell maybe a gang) of splicers as they try to survive in Rapture , and some where along the lines one would turn on the rest for some reason , i dont know why thats just how i picture it.
Edit : i aslo see it being semi CGI like 300 or something.