One idea could be exploring a world where Batman and Joker have to team up against a greater evil. Maybe a powerful alien force arrives in Gotham and they realize they need to combine their skills, even if reluctantly.
Some fanfictions might focus on the psychological aspects of their relationship. The Joker's insanity and Batman's unwavering sense of justice create a really unique dynamic. In slash fanfiction, this could be explored in terms of how they understand each other on a deeper level, perhaps a level that no one else can fathom.
In some fanfiction, the Female Joker might have a different origin story than the male one. For example, she could be a victim of a corrupt system who turns to chaos as a form of revenge. This could lead to an interesting dynamic with Batman, who is always fighting for justice in a more traditional sense. Batman may struggle with whether to help her or stop her, adding depth to their relationship in the story.
One idea could be that the deaged Joker, now with a child's body but his same devious mind, tries to manipulate the younger version of Batman into believing they are friends while secretly setting up traps. Another could be that the deaged Joker's actions cause Batman to re - evaluate his own origin story and how it might be related to this new, younger - but - still - dangerous foe. Or perhaps the deaged Joker forms an alliance with other deaged villains in Gotham, creating chaos that Batman has to stop.
One interesting plot idea could be a fanfiction where the Joker somehow comes back to life in Arkham Knight. Maybe he was only in a coma and wakes up with a new, even more deranged plan to take over Gotham. He could start by manipulating the inmates in Arkham again, creating chaos from within the asylum.
One idea could be that Rachel, instead of being in a typical relationship with Batman, has a series of mind - games with the Joker. The Joker tries to use Rachel to get to Batman in a more psychological way, like trying to make her question Batman's methods. Another plot could be that Rachel discovers a hidden connection between Batman and the Joker from their past, which changes her perception of both of them.
One plot idea could be that the Joker is in a situation where he's being hunted by a more dangerous criminal group, and Batman, despite their long - standing enmity, saves him because he realizes the chaos that would ensue if the Joker fell into the wrong hands. Another could be that the Joker has some information vital to Gotham's safety, and Batman has to rescue him from a hostage situation to get that information. Maybe the Joker is poisoned, and Batman has to find an antidote and save him in order to also stop the spread of the poison throughout Gotham.
One idea could be that the Joker kidnaps Batman to force him into a series of cruel 'games'. Maybe he wants Batman to choose between saving his closest allies or innocent civilians, testing his moral limits.
A plot could involve the Joker creating a new type of toxin that doesn't just affect people's physical state but also their perception of reality. Batman has to race against time to find the antidote while also dealing with the chaos in Gotham caused by the affected citizens. Or perhaps a story where the Joker discovers Batman's true identity in a very unexpected way and uses it to manipulate those closest to Bruce Wayne, forcing Batman to face a whole new level of personal and public danger.
Well, sometimes in such fanfiction, there are alternate universes. For example, a world where the Joker and Batman have to team up against a greater evil. This flips their typical dynamic on its head and makes for an exciting read.
One interesting story could be where Joker tries to turn Gotham against Batman in a very psychological way in the fanfiction. He spreads rumors and false evidence to make the citizens doubt Batman's motives.