One interesting storyline could be about Batman's wife getting involved in his crime - fighting activities secretly. She could use her own set of skills, perhaps in intelligence gathering or martial arts, to assist Batman from the shadows without his knowledge at first. This would add an element of surprise and a new dynamic to the Batman universe.
One interesting storyline could be a reimagining of Batman's first encounter with the Joker in the 1939 fanfiction context. Maybe the Joker starts as a small - time criminal and Batman has to figure out his true motives early on.
One interesting storyline could be John Constantine and Batman teaming up to fight a supernatural threat that's terrorizing Gotham. Maybe a demon is possessing the city's criminals, and Constantine's magic along with Batman's detective skills are needed to stop it.
One possible storyline could be Batman teaching Superboy how to better control his super strength. For example, in a training session, Batman sets up various obstacles that require Superboy to use just the right amount of force to get through. Another could be Batman training Superboy in detective skills. Superboy, with his super senses, has an advantage but needs to learn how to analyze clues like Batman. He might start by learning how to track down a simple thief in Gotham City under Batman's guidance.
One interesting storyline could be her struggle to live up to the Batman legacy while also forging her own identity. She might face villains who target her because of her relation to Batman. For example, she could be kidnapped by a new villain who wants to use her as bait to draw out Batman.
One interesting storyline could be Batman having to deal with a new villain who has the power to manipulate time while also trying to maintain his normal life as Bruce Wayne. This would add a whole new layer of complexity to his dual identity.
She could be from a different dimension in a fanfiction. In this new world, she has unique powers that help Batman in his fight against crime in Gotham. For example, she can communicate with animals, which gives Batman an edge when dealing with villains like the Penguin. Also, a story about their first date could be interesting. Was it a normal date or did it get interrupted by some villainous plot?
One possible storyline could be Kid Batman trying to prove himself to the full - fledged Justice League members. He might be given a small mission that seems simple but turns out to be full of unexpected challenges, like dealing with a new villain who has powers that can nullify the League's usual tactics. Another could be Kid Batman forming an unlikely alliance with a less - known member of the Justice League, and together they uncover a conspiracy within the League itself. Maybe there's a story where Kid Batman has to use his wits to save the day when all the other members are incapacitated.
One interesting storyline could be a team - up against a powerful new villain that neither can defeat alone. They have to combine their unique skills, like Superman's super strength and Batman's detective skills.
A cool fanfiction storyline could involve the Teen Titans being transported to an alternate version of Gotham where Batman is a villain. They have to figure out what went wrong and try to turn Batman back to the good side while also dealing with the chaos in this new Gotham.
Perhaps a story where Batman discovers a conspiracy within the Injustice League that involves mind - controlling the heroes. It could delve into his detective skills as he tries to uncover the truth and save his comrades. This would be full of mystery and action.