Well, in 'batman retires fanfiction', perhaps Batman realizes that the city has changed. The crime has become more political and less about the kind of street - level villains he used to fight. So he decides to retire and focus on using his Bruce Wayne identity to influence Gotham in a different way, like through philanthropy and reforming the city's infrastructure. He might also start mentoring young minds to fight crime in more legal and social ways.
One possible plotline could be that Darkseid is about to destroy Gotham or Earth in a way that Batman has no other option but to kill him. Maybe all of Batman's usual gadgets and allies are useless, and he has to use a kryptonite - like weapon (if it could affect Darkseid) to take him down.
In some 'batman x kagome fanfiction' plotlines, Kagome might be mistaken for a meta - human in Gotham. Batman starts to investigate her, and as he does, he discovers the truth about her world. This could lead to a series of adventures as they both learn from each other. Another idea is that a villain from one of their worlds somehow manages to cross over to the other. They then have to combine their skills and knowledge to defeat the villain. There could also be a plot where Kagome uses her knowledge of ancient magic to help Batman upgrade his technology in a unique way.
Commissioner Gordon would likely be a main character. He has been Batman's ally for a long time, and with Batman retiring, he'd have to face the challenges of keeping Gotham safe on his own. Also, Robin, if there is a current Robin in the story, could be a main character. He might be left with the big shoes to fill that Batman has left behind.
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.
A plotline could involve Kudo Shinichi being brought to Gotham, either by accident or some sort of experiment. There, he starts solving crimes in his own way, which catches Batman's attention. Batman might initially be suspicious of this new 'detective' in town and they could end up in a rivalry until they realize they can work together to take down a greater evil like the Joker who has some new, devious plan that affects both their worlds.
There could be a 'catwoman kills batman fanfiction'. A possible plot could be that Catwoman is in a situation where she has to choose between saving the city and Batman. She might think that sacrificing Batman is the only way to stop a greater evil from destroying Gotham. For example, if there's a doomsday device that can only be deactivated by Batman's death and she decides to go through with it.
One possible plotline could be Willie Watt using his powers for good this time. Maybe he teams up with Terry McGinnis (Batman Beyond) to take on a new threat in Neo - Gotham. They could combine their skills, with Batman's combat abilities and Willie's electrical powers.
One plotline could be that Copperhead is actually working undercover for an unknown force against the other villains in Arkham Origins. She could be secretly feeding information to this force while still maintaining her villainous facade among the other criminals.
One possible plotline could be that Meridian has some crucial information that Batman needs. Maybe Meridian has witnessed a crime related to one of Batman's archenemies. Batman chases Meridian to get that information before the bad guys do. He has to navigate through the dark alleys of Gotham, using his detective skills to track Meridian's every move.
One possible plotline could be that Batman is facing a new and extremely powerful villain. Maybe this villain has some sort of kryptonite - like substance that weakens Batman's physical and mental state. Another could be that Batman gets caught in a large - scale explosion while trying to defuse a bomb in Gotham. He could be severely injured and have to rely on his allies like Alfred and Robin to help him recover. And he might also face an internal struggle during his recovery, doubting his ability to continue protecting Gotham.