Their relationship could be portrayed as more of a friendship. Instead of the typical toxic relationship, they could be shown as two outcasts who support each other in their own strange ways. For example, Harley helps the Joker with his plans not out of love in a romantic sense, but because she values their friendship.
The Dark Knight's version of the Joker adds an extra layer of darkness to their relationship in fanfiction. Writers might explore how the Joker's chaos affects Harley even more in this darker context. For example, Harley could be torn between her love for the Joker and the realization of the horror he spreads in Gotham.
Harley Quinn could also be shown as a tragic figure. In the Dark Knight fanfiction, she could be a product of Gotham's corrupt system. Maybe she was failed by the very institutions that were supposed to protect her, which led her to a life of crime. This would add depth to her character and make her more than just a crazy villain.
The relationship can be portrayed as a journey. At first, the Joker and Harley might not know how to be parents. They could be clumsy and make mistakes. But as the baby grows, they start to adapt. The Joker might start to soften a bit, realizing that his actions could affect the baby's future. Harley would be the glue that holds the family together. She could be the one who arranges family dinners, even if it's just pizza in their hideout. The baby could also bring out a competitive side in them, like who can teach the baby the coolest trick or skill.
The Joker in 'The Dark Knight Rises' novel is presented as a character who thrives on disorder. He is shown to be extremely cunning, able to outwit many of his foes. His actions are often random and violent, yet there is a method to his madness. He uses his sharp intellect to create elaborate plans that are designed to disrupt the peace in Gotham and challenge Batman on both a physical and mental level. His character is a study in extreme human behavior and the consequences of a lack of moral compass.
Another way is to show a relationship full of regret. Anakin could be filled with remorse for how he treated Ahsoka in the past, but his dark side self still has the urge to do harm. Ahsoka, on the other hand, is conflicted between her love for the old Anakin and her need to oppose the current, dark Anakin.
Well, in some fanfictions, their relationship could start with Harley being the one in control. Instead of the Joker manipulating her, she could be the mastermind behind their schemes, which would be a complete role reversal. It would be interesting to see how the Joker reacts to this new dynamic.
Their relationship could be portrayed as a mentor - student kind of thing. Amy could be learning from Dark Super Sonic about the darker aspects of power and how to control it within herself. This would be a very different take from the usual romantic or friendly relationship.
Their relationship could develop through shared experiences. For example, they could go on heists together and during those times, Joker could start to see Harley in a different light. He might notice her intelligence and loyalty more and more.
The relationship could be shown as a mentor - mentee type in some fanfictions. Maybe the Dark Lord, in an alternate reality, decides to teach Harry some of his dark magic knowledge not to corrupt him, but to make him stronger to face other threats in the wizarding world. This would completely change the dynamic between them. They could also be shown as reluctant allies, forced to work together to stop a greater evil that is emerging in the wizarding world. This would involve a lot of tension and distrust between them at first, but gradually they could start to understand each other more.
The Joker's daughter could be depicted as a character with a lot of secrets. She may keep her true motives hidden from everyone, including her father's former associates. Her backstory could involve a difficult childhood, with the Joker being an unpredictable and often absent or cruel father figure. This could have led to her developing a tough exterior. In fanfiction, she could be shown as having a unique fighting style, perhaps incorporating elements of the Joker's erratic behavior into her combat moves, making her a formidable opponent in the dark alleys of Gotham.