It's a really wild idea. On one hand, Naruto has the power to take on the Joker. His chakra abilities like the Rasengan could potentially be a deadly weapon against the Joker. But on the other hand, the Joker is a master of psychological warfare, which Naruto may not be used to dealing with. In a fanfiction, it would be fascinating to see how the writer would make these two characters interact and how Naruto would ultimately defeat the Joker.
Well, it could start with the Joker causing mayhem in a village in Naruto's world. Naruto, being the hero, tries to reason with the Joker at first. But when he realizes the Joker is beyond reasoning, he uses his ninjutsu skills. He might start with some basic fire - style jutsu to test the Joker's limits and then progress to more powerful attacks like his Nine - Tails Chakra Mode attacks until the Joker is defeated.
Well, it could be that Naruto, with his ninja skills, sees the Joker causing chaos in his world which has somehow merged with the DC universe. He might use his Rasengan to take down the Joker. Maybe the Joker has a new evil plan involving stealing chakra or something, and Naruto just won't stand for it.
Well, one possibility could be that Naruto's powerful ninjutsu skills give him an edge. He might use his Shadow Clone Jutsu to confuse the Joker and then land a fatal blow with a Rasengan. Another aspect could be Naruto's strong sense of justice which drives him to take out the chaotic Joker in the Justice League universe.
Naruto's speed and strength would be a huge advantage. He can move faster than the eye can see with his ninja training, which would make it hard for the Joker to escape or set up traps. For example, if the Joker tries to throw one of his acid - filled flower things, Naruto could easily dodge and counter - attack.
One common theme is chaos. With Batman gone, Gotham descends into utter chaos as the Joker runs amok. Another theme is revenge. Sometimes the story is about someone seeking revenge for Batman's death. Also, the exploration of the Joker's victory is a theme, showing how he relishes in finally killing his nemesis.
There are some out there. Some explore the dark and twisted nature of such a scenario, delving into the psychological impact on Gotham and the other characters. However, the quality can vary greatly. Some might focus more on shock value while others try to build a complex narrative around it.
Often, there's a theme of justice being redefined. When Batman kills the Joker, it questions the traditional forms of justice in Gotham. It might also explore the theme of power. Does Batman become more of a tyrant or does he find a new way to use his power for good? And there could be a theme of redemption for Batman, as he tries to justify his actions to himself and others.
Maybe a family member of a police officer killed by the Joker. This civilian has a strong sense of justice and a personal vendetta. They are not bound by the same rules as the police and are more likely to take extreme measures. They have seen the destruction the Joker has caused to their family and will stop at nothing to get revenge.
One possible plot could be that the civilian has lost someone dear to the Joker's actions and decides to take revenge. For example, if the Joker killed the civilian's family member in a heinous crime, the civilian might spend months planning and training in secret to finally confront and kill the Joker. Another plot could be that the civilian is a vigilante who has had enough of the Joker's chaos in the city and decides to take matters into their own hands to protect the city.