One interesting storyline could be their team - up against a common powerful enemy. When they combine their unique skills, it creates exciting action sequences.
Perhaps a storyline where they have to resolve a conflict that is causing chaos in Gotham and Themyscira. This would involve them using their different perspectives, Wonder Woman's Amazonian wisdom and Batman's detective skills, to figure out the root of the problem.
Perhaps a storyline where Wonder Woman discovers a new aspect of her connection to Athena with Batman's help. They could go on a journey to an ancient land that was once under Athena's protection. Along the way, they face various challenges and enemies. This would allow for character development for both of them, as they learn more about each other and themselves in relation to Athena's legacy.
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 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 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.
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.