A third idea is that the OC is a mental health professional who gets the chance to treat the Joker in Arkham Asylum. Through their sessions, they learn about his past traumas and how they've shaped him into the monster he is. But the OC also has to be careful not to be manipulated by the Joker, which creates a tense and psychological story.
One idea could be that the OC is a psychologist assigned to study the Joker. As she tries to understand his mind, she gets drawn into his chaotic world. Another could be the OC is a new criminal in Gotham who catches the Joker's attention and they form an unlikely alliance against other villains. Maybe the OC has a special ability like mind - reading and the Joker wants to use it for his own gain.
One idea could be that the OC is a new villain in Gotham who catches the attention of both Joker and Harley. Maybe the OC has a unique power or skill that makes them a threat or an ally. For example, the OC could have the ability to manipulate minds, which Joker wants to use for his own evil plans, while Harley sees the OC as a potential friend who can understand her chaotic nature.
Another idea is to create a story where the OC is trying to find a cure for Joker Venom. They could be a scientist or a vigilante with a personal vendetta against the chaos it causes. They might face many obstacles, like being hunted by Joker's henchmen or not being trusted by the authorities.
One idea could be to create an OC who is a former victim of the Joker's madness and now has powers as a result. They could join the Young Justice team to seek revenge against the Joker. Another idea is an OC who is related to an existing character in the Young Justice universe and is lured into the Joker's chaotic world. For example, a relative of Robin who the Joker tries to corrupt.
One idea could be that the OC is a new villain in Gotham who catches the Joker's attention. He decides to use the OC to make Harley jealous, which then leads to a complex love - triangle - like situation full of drama and chaos.
One interesting Oc and Joker fanfiction could be about an Oc who is a detective trying to outwit the Joker. The Oc has unique skills and a backstory that makes the chase thrilling.
A Joker and Harley Quinn crossover with the Suicide Squad but in a different way. Instead of being part of the Squad, they form their own rebel group within the Squad and cause chaos in a more strategic and comical way. They could play mind games with the other members and the villains they are supposed to fight.
In this fanfiction, perhaps Naruto's transformation into a 'Joker - like' character could be a result of some extreme event. Maybe he experiences a great loss or betrayal that makes him question his values. Similar to how the Joker's backstory in the Batman universe shaped his character, Naruto could start to view the ninja world in a different, more cynical light and thus adopt the Joker's traits.
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.
For a joker and harley lemon fanfiction, you could write a story where they get lost in a magical forest. In this forest, they have to face their inner demons together. Harley could use her charm and wits to help Joker deal with his insecurities, and in the process, their relationship deepens. You could also explore a scenario where Joker is injured and Harley has to take care of him. This would show a different side of their relationship, a more tender side. Additionally, a story about them stealing a precious artifact from a powerful villain and how they celebrate their victory with some passionate moments could be interesting.