One possible plotline could be that Batman is tricked into joining the Dark Side. Maybe the Joker uses some elaborate mind - control device or false information to make Batman believe that the Dark Side has the right way to bring true justice to Gotham. For example, he could be made to think that by using more extreme and violent methods, he can finally stop all the crime in the city once and for all.
Perhaps the female Naruto joins Orochimaru because she wants to learn more about the secrets of the ninja world that she believes Orochimaru can unlock. She may be curious about the darker aspects of ninjutsu that are not taught in the normal ninja academies. But then she discovers that Orochimaru is using her for his own immoral experiments. She then teams up with some of Orochimaru's former followers who also want to break away and they plot to take him down from the inside.
Perhaps Harry discovers a dark secret about the side he has been fighting for. This discovery makes him feel betrayed and he turns to Tom thinking that Tom offers a different kind of solution. It could be that Tom presents himself as a reformed person or has a convincing argument about a new order in the wizarding world that Harry, in his disillusioned state, starts to believe. Then, the story could follow how Harry's relationships with his old friends deteriorate as he gets closer to Tom's side.
Well, perhaps Naruto and Sasuke are forced to join Orochimaru. Maybe Orochimaru has some kind of leverage over Konoha that makes them comply. In this fanfiction, they could start to learn unique jutsus from Orochimaru but also have to constantly resist his more evil intentions. They might also meet some interesting characters within Orochimaru's lair.
One possible plotline could be that Rey finally succumbs to the dark side under Kylo Ren's influence. He might train her in the ways of the dark side, leading them to rule the galaxy together with an iron fist. Maybe they start by taking over the First Order and using its resources to spread fear and chaos.
Well, in 'batman retires fanfiction', perhaps Batman realizes that the city has changed. The crime has become more political and less about the kind of street - level villains he used to fight. So he decides to retire and focus on using his Bruce Wayne identity to influence Gotham in a different way, like through philanthropy and reforming the city's infrastructure. He might also start mentoring young minds to fight crime in more legal and social ways.
A plot could revolve around a dark force corrupting both Naruto and Hinata. This force might be an ancient evil that has been dormant for years. As Dark Naruto and Dark Hinata, they might target their former friends. Naruto could use his shadow clones in more menacing ways, and Hinata could use her Gentle Fist techniques to cause harm instead of protect. They could form an alliance with other outcasts or villains in the Naruto universe.
One way could be to explore the idea of a great betrayal. Maybe a powerful Sith Lord manipulates events in such a way that Anakin and Obi - Wan both feel they have no other choice but to side with the Dark Side. For Anakin, it could be something related to his fear of losing Padmé, and for Obi - Wan, perhaps the Jedi Order's actions against him make him lose faith. The story could start with a series of small incidents that gradually push them towards the Dark Side.
One possible plotline is that Superman is tricked by an alien race into believing that Earth is a threat to the universe. So, he decides to use his powers to stop Earth from 'causing harm', which is really him going to the dark side as he's misinformed.
One possible plotline could be that the dentist has a secret addiction to causing pain rather than relieving it. Maybe they use their position to experiment on patients in unethical ways.
One possibility could be that Batman's sense of justice becomes warped. He might start using more extreme and unethical methods to 'clean up' Gotham. For example, he could be more brutal with criminals, foregoing his usual no - kill rule. His relationships with his allies would also be severely strained. Alfred would be deeply disappointed and the Bat - family might split apart as they try to oppose his new, darker path.