Spoilers For The Next Sin City Movie

Frank Miller has confirmed that he is working on new Sin City storylines. The following have been mentioned:

Nancy avenges Hartigan's death by killing remaining members of the Roark family. Miller says this will show "a whole new side of Nancy." This story has been confirmed as one of the main stories for the planned movie Sin City 2.
A prequel story about Hartigan
A story with a brand new character
Although there has been much fan speculation on specifics (as well as how many stories Miller will publish in total), few details have been verified thus far.