One idea could be that Harley Quinn tries to reform herself to be worthy of Batman's trust. Maybe she starts secretly helping Batman take down other villains in Gotham. Another could be a situation where they are forced to team up against a common, extremely powerful enemy, like a new supervillain who has a plan to destroy Gotham. And a third idea is that Harley Quinn has a change of heart about the Joker after seeing how Batman is constantly fighting for good, and she begins to see Batman in a new light, leading to a complex relationship between them.
Well, in Harley Quinn x Batman fanfiction, a great plot could be that Harley Quinn discovers a secret from Batman's past that puts her in a position to either expose him or help him keep it hidden. This could lead to a lot of tension and character development. Or perhaps there's a story where Harley Quinn gets amnesia and Batman has to deal with this new, more innocent version of her. He could try to use this opportunity to turn her fully to the side of good. Also, a plot where Harley Quinn and Batman are stuck in an alternate dimension together and have to rely on each other to get back to their own world would be really exciting.
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.
One plot idea could be that Harley Quinn is being hunted by a new super - villain in Gotham who wants to use her knowledge of the Joker's operations against him. Batman, despite their past differences, decides to save Harley because he realizes she could be a key in preventing a much larger threat to the city. He manages to rescue her from a dangerous trap set by the new villain and in the process, Harley starts to see Batman in a new light, not just as an enemy but as someone who actually cares about her well - being.
Well, a great plot could involve Harley Quinn's chaotic nature clashing with Bane's more methodical and brute - force approach. For instance, they are forced to work together on a heist. Harley wants to do things her wild way while Bane is trying to stick to a plan. As they argue and work through their differences, they start to respect each other. Also, perhaps they find out that they both have a vendetta against the same corrupt Gotham official, which brings them closer.
One idea could be that the OC is a new villain in Gotham and Harley Quinn takes an immediate liking to their chaotic style. They team up to pull off a heist on one of Gotham's richest, with Harley's unpredictable nature and the OC's unique skills making for an exciting adventure.
One interesting plot could be Harley Quinn trying to reform and Batman being skeptical at first. For example, she might start to help Batman take down some minor villains as a way to prove herself. Another plot could involve a team - up where they have to face a new, powerful enemy that neither could defeat alone. Maybe a villain who has the power to manipulate minds and they have to work together to resist the control.
A great plot idea for this fanfiction could be that Harley Quinn is haunted by the memories of her time with the Joker during the Arkham Knight scenario. Batman, noticing her distress, tries to help her. As they interact more, Harley starts to see Batman in a new light and decides to assist him in his fight against the Arkham Knight. Along the way, they face many challenges, like dealing with Harley's old gang who don't trust her new alliance with Batman, and also the Arkham Knight's devious traps. This would make for a complex and engaging fanfiction story.
There could be a plot where Poison Ivy discovers a new type of plant that has the power to make people tell the truth. Harley gets curious and tries it on some of the bad guys in Gotham. Meanwhile, Ivy is worried that this new plant could attract unwanted attention from the government or other powerful forces. They have to decide whether to keep it a secret or use it for their own purposes.
One idea could be Harley Quinn getting a fresh start in the Batman Beyond era. Maybe she tries to reform and uses her unique skills to help the new Batman. She could team up with him to take on a new villain that's causing chaos in the future Gotham.
In some batman x harley fanfiction, there might be a scenario where they have to team up against a common enemy. This forces them to put aside their differences and work together. For example, they could be facing a new supervillain who is a threat to Gotham and they find that their combined skills are necessary to defeat the foe. It's an exciting concept as it shows a different side of their relationship, not just the typical hero - villain dynamic.
A plot twist might be that Harley Quinn has a change of heart not just about the Joker, but also about Gotham's power structure. She decides to help Batman not just against the Joker, but also against some corrupt elements in Gotham that Batman has been trying to expose. So, instead of just being a one - off ally against the Joker, she becomes a long - term partner in cleaning up Gotham.