A fanfic could explore a scenario where Batman and Superman swap powers. Imagine Batman having super strength and flight, while Superman has to rely on gadgets and his wits in Gotham. It would be a great way to show how they adapt to different situations and powers.
Another interesting concept for a batman x superman fanfic could be set in an alternate universe where they are enemies from the start. Maybe due to a misunderstanding or some outside force manipulating them. The story could then be about how they eventually come to realize the truth and work together.
One idea could be a story where Batman and Superman have to team up to stop a powerful new villain who is using kryptonite - based technology. This would put Superman at a great disadvantage and force Batman to use his detective skills to find a way to neutralize the threat.
One idea could be a story where Batman and Superman have to team up to stop a new villain that has the power to manipulate time. This would force them to put aside their differences and really work together in a way they haven't before.
Another aspect could be the formation of new alliances. Since Superman is evil, Batman might have to team up with other unexpected characters. For example, he could join forces with Lex Luthor, who usually opposes Superman. This would create a very different power dynamic and storyline, as they have to put aside their differences to face the greater threat of an evil Superman.
A cool plot could be that a powerful alien race arrives on Earth and offers a deal to the world leaders. Batman is suspicious of their motives and starts investigating. Superman, on the other hand, is more open - minded at first. This causes a rift between them. As Batman uncovers more evidence of the aliens' true intentions, he has to convince Superman. In the end, they join forces to stop the aliens from taking over the Earth.
Perhaps Batman has teamed up with some of Superman's other foes who have knowledge of his weaknesses. For example, Lex Luthor could provide Batman with technology or information that gives him the edge. Also, Batman might use psychological warfare on Superman, playing on his insecurities about being an alien among humans.
One idea could be that Batman discovers a new Kryptonian weapon that Superman unknowingly has a weakness to. This weapon was hidden on Earth long ago by a rogue Kryptonian. Batman then has to decide whether to use this knowledge to defeat Superman or find a way to help Superman overcome this potential threat together.
One idea could be a story where Batman and Superman's kids team up to solve a mystery in Gotham City. Maybe they find a strange device that's causing chaos and they have to use their combined skills, like Batman's detective skills and Superman's super strength, to figure it out.
One idea could be the two heroes having to hide their relationship from the public while still saving the world together. They might face challenges like dealing with their different approaches to justice within the context of their romance.
One interesting Batman Superman fanfic could be about them teaming up to face a new, extremely powerful villain that neither could defeat alone. For example, a villain with the power to manipulate time. In the story, Batman's strategic mind and Superman's superpowers combine in unique ways to outwit the foe.
One idea could be that Superman has to team up with the Furies to stop an otherworldly threat that is too powerful for him to handle alone. Maybe a powerful alien race is invading Earth and the Furies' unique skills and abilities complement Superman's powers in the fight.
Another plot could involve a mad scientist who creates a device that can drain the powers of superheroes. He first targets Superman, but Spiderman gets involved when he sees the chaos in Metropolis. As they try to stop the scientist, they discover that the device has a side effect of giving the heroes new, unexpected powers for a short time. They have to quickly learn to control these new powers while also dealing with the main threat.