Ever since Sin City opened well earlier this year, a sequel has been in the works – but details about the plot have been under wraps. However, a few lines have cropped up on Wikipedia, apparently from Sin City creator and co-director (with Robert Rodriguez) Frank Miller, that suggest what might be happening next.
Most importantly, Miller has apparently confirmed that he is working on entirely new Sin City storylines. These apparently include Nancy (Jessica Alba) avenging Hartigan's death by killing the remaining members of the Roark family. Miller says that this will show "a whole new side of Nancy", and is one of the main stories in the movie.
There will also be a prequel story involving Hartigan (Bruce Willis), and a story about a brand-new character.