A great storyline might involve a situation where they are forced to trust each other completely. Maybe there's a magical artifact that is corrupting the minds of the heroes and they have to rely on each other's unwavering wills. Batman has his strong sense of justice and Wonder Woman her Amazonian values. Their journey to resist the artifact's influence while also trying to destroy it could be very engaging.
One interesting storyline could be them teaming up to solve a mystery that involves both Gotham and Themyscira. Maybe a powerful ancient artifact has been stolen from Themyscira and is being used to create chaos in Gotham. Batman's detective skills and Wonder Woman's knowledge of the artifact's origin would make for an exciting adventure.
There are fanfictions where Wonder Woman and Batman develop a deep emotional bond. For example, in a story, they might be stranded on an alien planet during a Justice League mission. They have to rely on each other to survive, and in the process, they learn more about each other's pasts and fears, which brings them closer together.
Maybe in a fanfiction, they kiss during a moment of extreme danger. For example, Batman is about to sacrifice himself to save Wonder Woman and she stops him with a passionate kiss. This could be the start of a new relationship dynamic where they start to question their previous boundaries and how they feel about each other.
One interesting storyline could be Batman and Wonder Woman teaming up to take on a new, powerful villain that neither could face alone. This villain could be a magical threat that requires Wonder Woman's knowledge of the mystical and Batman's strategic mind.
One interesting storyline could be the two superheroes uniting to fight a new, powerful villain that threatens both Gotham and Themyscira. Their marriage would symbolize the union of two different worlds, and they'd use their combined skills and resources to protect everything they hold dear.
One great storyline could be Batman and Wonder Woman teaming up to face a new, powerful villain in the JLU universe created by a fanfic writer. Maybe a villain who can manipulate time, forcing them to rely on each other's unique skills. Batman's detective skills and strategic mind combined with Wonder Woman's combat prowess and divine powers would make for an exciting adventure.
One interesting storyline could be their child having to hide their powers in a normal school. They'd struggle to fit in while also having the responsibility of their superhero heritage. Another could be the child getting kidnapped by a villain who wants to use their unique genetic makeup for evil.
One interesting storyline could be the son's struggle to balance the very different legacies of his parents. He might have Batman's brooding nature and tactical mind, while also having Wonder Woman's sense of justice and connection to the gods. This internal conflict could lead to some great character development.
One possible storyline could be that they are facing an extremely powerful and ancient evil. This evil has the power to corrupt the very essence of the world. Batman and Wonder Woman, in their attempt to stop it, find themselves in a situation where they have to sacrifice their lives. Maybe they are lured into a trap where the only way to prevent the evil from spreading across the universe is for them to use their combined powers in a self - destructive way.
A great plot could be a love story between them in the fanfiction world. They come from different backgrounds, she's an Amazonian princess and he's a brooding vigilante from Gotham. Their relationship could be filled with misunderstandings at first but then develop into a deep connection. For example, they might initially clash over their different approaches to justice but later find common ground and fall in love.