Without the Joker, Harley Quinn will become the main criminal authority of the District of Columbia

Attention: contains spoilers for the game DC vs. Vampires: Killers #1!
As vampires flood the DC universe, Harley Quinn steps into an impressive new role that shows just how much the Joker held her back. When she was his villainous girlfriend, the Joker constantly scolded the former psychiatrist, making him think that she was useless without him. Harley’s turn to heroism allowed her to truly break the toxic bond with the Clown prince of the underworld, but it turns out that if she had pursued a criminal career without him, she would have ruled the underworld of Gotham.
In the ongoing DC vs Vampires, Earth’s superheroes have been defeated by the Vampire King— the new title of Dick Grayson, also known as the Avenger, known as Nightwing. Calling Wonder Woman, Superman, Shazam and Green Lantern his slaves, Nightwing began a vampire takeover of the Earth, making Gotham his main possession. However, vampires still need humanity to feed, which means that people like Harley have a chance to start a new life in the shadows.
Related: The first Harley Quinn from DC was in love with Batman, not the Joker
In a one-time tie-in DC vs. Vampires: Killers — from Matthew Rosenberg, Mike Bowden and Eduardo Mello — the world is now officially overrun with vampires who have successfully blocked Gotham, preventing anyone from entering or leaving the city. Most of the characters (especially the heroes) are out of action, including the Joker, who was initially suspected of being the Vampire King, but who turned out to be dead in DC vs. Vampires #6. There aren’t many survivors left, but Harley Quinn is one of them. In the absence of the Clown Prince of the underworld, Harley has taken over Gotham City as his new kingpin. “I think I’m running Gotham’s underworld now,” she says. “These are the rules.” Harley Quinn gathers a gang consisting of Catwoman, Solomon Grundy and Clay Face and successfully makes deals with the Mad Hatter before shooting him when he becomes a problem.
When the Joker disappeared, but the other Gotham villains are still in the game, Harley Quinn became the most dangerous woman in Gotham. This is especially impressive considering how much the vampires have locked up Gotham City. Harley’s planning, leadership and lethality allow her to thrive in seemingly impossible conditions, showing that the Joker not only failed to teach her his chaotic crimes, but also prevented Harley from reaching his true potential.
While the Joker’s plans have always had a hero at every turn to thwart his dastardly plans, Harley Quinn has seized the perfect opportunity to become a crime boss now that most of the heroes have either been killed or disappeared. While later stories in DC history have established how being the Joker’s sidekick held back her full potential as a hero, this story shows how this time in her life held back Harley Quinn from her potential as a true supervillain-perfect for surviving in a world ruled by the living dead.