On a recent promotional tour for Sin City's new DVD release, directors Robert Rodriguez and Frank Miller (also the writer, in case you somehow didn't know) alongside guest director Quentin Tarantino discussed the upcoming sequel. The sequel, as it is by now well known, will be partially based on A Miller's a "Dame To Kill For."
However, they also want to provide a movie that the audience won't already know the story for, so to that end, Miller is penning new stories to entertwine with the existing framework. New characters are also a distnict possibility, but the creative team refused to reveal many details for now. The possibility for a new book based on this material is apparently quite real, as well.