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.
Another storyline could be a betrayal within the superhero community. A powerful figure they trust turns against them. This person manages to set up a series of events that lead to Batman and Wonder Woman's deaths. For example, they could be framed for a crime they didn't commit, and while trying to clear their names, they are targeted by other heroes who have been misled. Eventually, they are overpowered and killed.
Perhaps there is a prophecy that foretells their deaths as the only way to save the world from a coming apocalypse. They initially resist this fate, but as the signs of the apocalypse start to become more and more evident, they come to accept it. They go on one last mission together, facing hordes of demons or some otherworldly creatures. In the end, they give their lives to close a portal or destroy a powerful artifact that is the source of the apocalypse.
A possible storyline could be that Wonder Woman comes to Gotham to investigate some strange occurrences that seem related to her world. She teams up with Batman, and they discover a cult that is trying to summon an ancient evil. Along the way, they learn to respect each other's methods.
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.
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.
A really interesting plot could be that the child is kidnapped by an old enemy of either Wonder Woman or Batman. The kidnapper wants to use the child to get revenge on the heroes. The child then has to use their wits and the powers they've inherited to escape. Along the way, they might make new friends or discover secrets about their parents' pasts that help them in their journey.
One possible plotline could be that Batman sacrifices himself to save Wonder Woman from a powerful enemy. Maybe he takes on a self - destructing device or holds off a horde of villains while she escapes. Another could be that he is poisoned by a new and deadly toxin, and in his final moments, he imparts some crucial knowledge or advice to Wonder Woman.