The tragedy behind this is that there was an actual Sin City game being written by Frank Miller that was cancelled in 2008. It didn't get very far in production, but the fact that Miller was involved at all would have meant that the game would have been more graphic novel flavored than movie tie-in flavored.

Marv would be the obvious choice for the protagonist, but the whole cast would be just as easy to make interesting and playable characters in a Sin City title. Think Grand Theft Auto meets Max Payne, and keep the whole thing monochromatic and give fans of both the comic and the film an open world bloodbath that they deserve.