One idea could be a story where Harley Quinn tries to break free from the Joker's control completely and starts her own life of crime without him. She could form her own gang and prove that she's more than just the Joker's sidekick.
A plot could revolve around the Joker and Harley Quinn in an alternate universe where they are normal people. They meet in a mental institution not as patient and doctor, but as two lost souls trying to find their place in the world, and then their relationship develops from there.
Perhaps a fanfiction where Harley Quinn gets amnesia. The Joker then has to deal with a Harley who doesn't remember him or their relationship. He could try to make her fall in love with him all over again, but she starts to see his true nature more clearly without the blind love she had before.
One idea could be that Harley Quinn and Joker have a baby, and the baby inherits their chaotic and unpredictable nature. Maybe the baby starts causing mayhem in Gotham from a very young age, and the other villains and heroes are all shocked and don't know how to deal with this new situation.
One idea could be Harley Quinn trying to break free from the Joker's control completely within the Suicide Squad setting. Maybe she starts to form real friendships with the other Squad members, like Deadshot, and they help her see that she doesn't need the Joker's madness in her life.
One idea could be that the baby has the Joker's chaotic nature but Harley's charm. For example, the baby might play pranks on the henchmen in a really cute yet mischievous way. Another could be that the baby has some special powers, like being able to make people laugh uncontrollably just like the Joker, but also has Harley's acrobatic skills at a very young age. Maybe the baby could be the key to a new, more 'family - oriented' life for the Joker and Harley, leading them to try and reform for the sake of their child.
One idea could be exploring their relationship before Harley fully embraced her criminal side. Maybe show how the Joker manipulated her in the early days. Another could be a story where they have to face a common enemy in the Suicide Squad universe that forces them to re - evaluate their relationship. Or perhaps a story where Harley starts to break free from the Joker's control and how he reacts to that.
One idea could be a fanfiction where Harley Quinn tries to break free from the Joker's control completely. In this story, she starts to realize that his love is more about possession. She could meet new allies who help her see her self - worth outside of being the Joker's sidekick.
One plot idea could be that Spiderman and Harley Quinn team up to take on a common enemy in Gotham. Maybe a new villain emerges who has technology that can disrupt both of their cities, so they have to combine their skills. Another could be a story where Harley Quinn, being a bit of a wild card, accidentally gets transported to Spiderman's New York and they have to figure out how to get her back while also dealing with the chaos she causes.
Maybe Harley Quinn discovers some dark secret about Nightwing's past that she uses to blackmail him into helping her with a heist. But as they work together, they start to understand each other better. Nightwing realizes Harley isn't all bad and Harley sees that Nightwing has his own demons. In the end, they might even become friends or something more.
One idea could be that Harley Quinn and Deadpool team up to take on a common enemy who has been causing chaos in both their worlds. They could start off as reluctant partners, with Harley being annoyed by Deadpool's constant chatter and Deadpool finding Harley's craziness a bit much at first. But as they fight side by side, they learn to appreciate each other's skills and form an unlikely friendship.
One plot idea could be that Deadpool and Harley Quinn team up to take on a common enemy, like a new super - villain in Gotham who has a connection to some of Deadpool's foes. They could start off not getting along because of their different personalities, Deadpool's fourth - wall - breaking and Harley's chaotic - cute nature. But as they face challenges together, they learn to work as a team and even become friends.
One idea could be Harley Quinn trying to reform herself and Batman being reluctantly drawn into helping her. Maybe she has some crucial information about a new villain in Gotham and in exchange for her safety and a chance at a new life, she offers to work with Batman.