Well, perhaps a plot where Catwoman is injured during one of her heists and Batman finds her. Instead of turning her in, he takes her to his secret lair to nurse her back to health. As she recovers, they start to have long conversations about their lives and motives. This deepens their connection and eventually leads to a romantic relationship full of tension and passion, with Catwoman teasing Batman and him trying to keep his cool while being drawn to her allure.
One plot idea could be that Catwoman is on a heist and Batman tries to stop her as usual, but during the encounter, they share a moment where they realize their mutual attraction. They start secretly meeting up, away from the chaos of Gotham's crime world, and gradually fall in love. Another idea is that they are forced to work together to take down a common enemy much bigger than their usual foes. In the process, they see different sides of each other and their relationship deepens.
One idea could be that Batman and Catwoman have a daughter who discovers her parents' secret identities. She could struggle with the duality of their world and try to find her own place in it. Maybe she has her own unique set of skills that combine elements of both Batman and Catwoman.
One idea could be a story where Batman and Catwoman team up to take on a new, more powerful villain in Arkham. Maybe this villain has the ability to control the minds of the inmates, creating chaos. They'd have to use their unique skills, Batman's detective work and combat abilities, and Catwoman's agility and thieving expertise to stop the threat.
A possible plot could be that Lex Luthor teams up with some of Catwoman's foes to frame her for a major crime. Superman is initially sent to capture her but as he digs deeper, he realizes she's been set up. So he decides to help her clear her name. During this process, they have to hide from both the law and their enemies. In the close quarters and the shared danger, they form an unexpected bond.
In this fanfiction, it could be interesting if Catwoman discovers a secret about the Arkham Knight that Batman has overlooked. She could use this knowledge to her advantage, either to make a deal with the Arkham Knight or to help Batman. For example, she might find out that the Arkham Knight has a weakness related to a certain location in Gotham that was important to his past. Catwoman could then lead Batman there to take down the Arkham Knight.
Perhaps in the Young Justice universe, Catwoman discovers a secret about Batman's past that puts her in a moral dilemma. She could either use this information to her advantage or help him keep it hidden. As they interact more because of this, feelings start to develop between them that they didn't expect. They have to navigate their relationship while also dealing with the threats that Young Justice faces.
One idea could be that Batman and Catwoman kiss during a moment of truce while they are teaming up to take down a greater threat in Gotham. Maybe they are on a rooftop, surrounded by the chaos of the city below, and in that moment of shared purpose, their feelings surface and they share a passionate kiss.
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.
An interesting idea is to write a story where Catwoman discovers a secret about Batman's past that even he doesn't know. This could lead to a strange alliance between them as they both try to figure out what the discovery means. It would also add a new layer to their relationship.
One plot idea could be that the reader is a detective who is initially on Catwoman's trail. But as they interact more, they realize that Catwoman has her own code of ethics and they end up teaming up to take down a bigger bad in Gotham. It could be a story of mutual respect and understanding growing between them.