In 'the joker fan fiction', a frequent theme is the Joker's unpredictability. Writers love to play with how he can suddenly change his plans or actions. The theme of power is also there. He is always trying to gain power in his own twisted way, over Gotham or over his enemies. And the theme of redemption sometimes pops up, even though it's rare, but it makes for an interesting take on his character.
The relationship can be explored by showing how they are two sides of the same coin. For example, in a fan fic, Batman could be faced with a situation where he starts to understand the Joker's chaotic view of the world a bit more, maybe due to some extreme circumstances. This would make him question his own motives and methods.
Fan fiction about the Joker's origin is a really interesting concept. Fans often write these to fill in the gaps or create their own interpretations of how the Joker came to be. It could involve exploring his psychological state before becoming the Joker. Maybe he was a normal person who suffered some sort of extreme trauma that pushed him over the edge. Or it could be about his relationships with other characters in the Batman universe during his early days that influenced his development into the Joker.
The significance can be the exploration of different character dynamics. In fan fiction about the Joker's death in the Batman universe, it allows fans to imagine how Batman would be affected. It could show a new side of Batman, perhaps a more conflicted or less morally certain version as he deals with the death of his archenemy.
There are fan fictions that explore their relationship in a more comical way. For example, Harley and Joker get into all sorts of wacky misadventures in Gotham City, with Harley constantly trying to 'fix' the Joker's schemes in her own chaotic way. These stories often have a light - hearted tone and are full of humor.
Obviously, Harley Quinn and the Joker are the most popular. They are the main focus of this fan fiction genre. Their complex and twisted relationship draws a lot of fans.
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.
First, you need to understand the characters well. Know their personalities, strengths, and weaknesses. For example, Joker is often seen as a bit of a rebel, while Makoto is more of a rule - follower. You can use this contrast to create interesting interactions between them.
You can start by looking on fan - fiction websites like Archive of Our Own (AO3). It has a large collection of fan - created works, including many Harley Joker fan fictions. Just use the search function and type in 'Harley Joker' and you'll get a bunch of results.
One idea could be that they go on a secret mission together outside of their usual group activities. They face various challenges and in the process, their relationship deepens as they rely on each other's skills.