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.
Another could be a story where Batman teams up with other early DC characters in a way that was not explored in the original 1939 comics. For example, he could work with Commissioner Gordon in a more in - depth way, like they are solving a series of connected crimes that are plaguing Gotham. The fanfiction could explore their relationship and how they build trust in each other.
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.
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.
One interesting storyline could be Poison Ivy trying to convert Batman to her environmentalist views. She might use her powers to show him a future where Gotham is destroyed by pollution and convince him that her extreme methods are necessary. Another could be a story where Batman has to rescue Poison Ivy from a more dangerous villain who wants to use her powers for evil. This would put them in an unexpected alliance situation.
One interesting storyline could be about Batman having to face a new villain that was created by the combination of the fear toxins from Scarecrow and the technology of Hugo Strange. This new villain has the ability to manipulate people's minds on a large scale and Batman has to use all his detective skills and combat abilities to stop it.
In some Mpreg fanfictions, there could be a storyline where Batman and Superman have to figure out the genetics of their unborn child. Given their different origins - one being a Kryptonian and the other a human with no super - powers originally - it would be fascinating to see how their child might inherit or combine their abilities. There could also be a plot where they have to deal with time - traveling villains who try to prevent the birth of their child for some evil purpose.
One interesting storyline could be Catwoman stealing a valuable artifact that has a connection to Batman's past. This forces Batman to track her down, but as he does, he discovers that Catwoman has her own reasons for taking it that are more complex than simple theft. Another could be a situation where Catwoman and Batman are forced to team up against a greater threat in Gotham, like a new super - villain who is out to destroy the city. And a third one might be a story where Catwoman tries to frame Batman for a crime he didn't commit, leading to a tense cat - and - mouse chase between them.
One interesting storyline could be Joker devising a new, elaborate plan to take over Gotham City that involves some wacky and unique traps. Batman, of course, has to use his Lego - themed gadgets and skills to stop him. Another could be a story where Joker and Batman are forced to team up against a greater threat, like a powerful alien invasion in the Lego universe. Maybe they start to understand each other a little better during this time.
In some 'batman battle of the cowl fanfiction', there could be a storyline where the villains take advantage of the chaos during the battle. They might form new alliances or come up with more devious plans. The fanfiction could explore how the remaining heroes in Gotham, not just those directly related to Batman, respond to this new threat.