A fanfic could explore what would happen if Catwoman found out Batman's true identity before he was ready to reveal it. Maybe she uses this knowledge to try and protect him from a threat that is specifically targeting Bruce Wayne, and in the process, their relationship takes an unexpected turn.
In a fanfic, Catwoman and Batman could be in an alternate universe where they are actually partners from the start. They work together to keep Gotham safe, but also have to deal with the public's perception of their relationship, which is a mix of admiration and suspicion.
There are also fanfics that put Batman and Catwoman in alternate universes. In some of these, they could be in a world where Gotham has a different set of rules or they themselves have different powers. It's really fun to read these and see how the relationship between them changes in these unique settings.
Maybe there's a fanfic where a child is accidentally caught in the middle of a battle between Batman and some of his rogues gallery, and Wonder Woman steps in to save the child. The child then becomes sort of a mascot for the two heroes, inspiring them to work together more closely and also showing the softer side of the often - brooding Batman.
A great plot could be that there's a masked vigilante hunter in Gotham who targets both Batman and Catwoman. They have to hide from this hunter while also trying to figure out who it is. Along the way, they take refuge in an abandoned part of the city and during this time of forced proximity, they have deep conversations about their motives, their pasts, and what they really want for the future of Gotham.
There's a fanfic called 'Midnight Rendezvous'. In this one, the setting of Gotham at night is vividly described. The encounters between Batman and Catwoman are filled with tension and passion. The writer has a unique style of writing their dialogues, which makes it a very engaging read.
A popular 'Batman and Catwoman Fanfic' is 'The Thief and the Dark Knight'. It's set in an alternate Gotham where Catwoman is on a mission to steal a precious artifact, and Batman has to stop her while also dealing with his growing feelings for her. There are also some really good ones on Archive of Our Own, like 'Gotham's Lovers' which shows their relationship from the perspective of the citizens of Gotham.
In a different scenario, Batman is transported to the realm of Athena in some sort of magical accident. There, he has to prove himself worthy to her. He learns from her about justice in a different light, not just the street - level justice he's used to in Gotham. When he returns to Gotham, he has a new perspective on how to deal with crime and corruption.
One plot idea could be that they decide to take a break from their crime - fighting lives. They could focus on preparing for the baby, like finding a safe place to live away from their enemies. Maybe they would have to deal with villains who want to use the pregnancy against them.
One popular theme is their romantic relationship. Writers often explore how they fall in love despite being on different sides of the law. Another is their team - up against other villains. They both have unique skills, and a fanfic could focus on how they combine their abilities to take down a powerful foe. Also, there are stories about Catwoman's backstory and how Batman comes to understand her more deeply through that.
One idea could be a story where Catwoman steals a valuable item during the chaos of Gotham's post - Bane takeover in The Dark Knight Rises. Batman has to track her down, but as he does, they start to realize their feelings for each other are more complex than just hero - villain. They could end up teaming up against a new threat that emerges from the ashes of Bane's rule.
One common storyline is his coming - of - age journey. He starts to discover his powers and abilities, much like how Batman and Catwoman came into their own. For instance, he might face a series of challenges in Gotham that force him to use his wits and skills.