Speaking on the Millarworld panel at Kapow! Comic Con, the writer revealed that he is working on the project with Kick-Ass collaborator John Romita Jr.
Millar said that the series would contain all the things that he had wanted to include in Kick-Ass 2 but couldn't find the space.
He said that there were no plans to produce a Hit Girl movie. He likened the idea to creating a spinoff film starring Han Solo, saying that some characters work better as part of an ensemble.
Hit Girl first appeared in the first series of Kick-Ass, a comic about a boy who decides to become a real-life superhero.
Chloe Moretz's portrayal of the foul-mouthed, violent little girl in the movie raised some controversy over her swearing during the film. She received the 2010 Digital Spy Movie Award for 'Best Actress' for the role.
Millar and Romita Jr are currently working on Kick-Ass 2. Work has also begun on Hit Girl.