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.
Another aspect could be his relationship with other characters. If Naruto is the Joker, his interactions with Sasuke could be really interesting. Sasuke, who is often serious and brooding, might be constantly frustrated by Naruto's new - found chaos. Naruto could play mind games with Sasuke, which would be a complete shift from their usual dynamic of rivalry and friendship.
One idea could be that Naruto somehow gets trapped in a strange dimension where the rules are set by the Joker, and Ino has to use her mind - reading abilities to figure out how to rescue him. Another could be a scenario where the Joker is after some powerful ninja art in the Naruto universe and Ino and Naruto team up to stop him. Maybe there could also be a story where Naruto's positive attitude starts to influence the Joker in a strange way while Ino is caught in the middle, trying to keep Naruto safe and also deal with the unpredictable Joker.
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 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.
One plot idea could be the Joker trying to prove his love to Harley in a really extreme way, like taking over Gotham City and dedicating it to her. It shows his over - the - top and crazy way of loving.
One idea could be a story where Harley Quinn tries to break free from the Joker's control completely and starts her own life of crime without him. She could form her own gang and prove that she's more than just the Joker's sidekick.
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 interesting plot idea could be that the Joker somehow gets transported to the RWBY universe. He might start causing chaos in Vale, tricking the villains and heroes alike. For example, he could convince Roman Torchwick to do his bidding in a more extreme way, which would then draw the attention of Team RWBY. Another idea is that he teams up with Cinder Fall, but his chaotic nature eventually causes their alliance to crumble as he doesn't follow her grand plans in the way she expects.
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 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.