A possible plotline is that the baby is a key to unlocking an ancient secret in Gotham. Bruce Wayne has to protect the baby from various villains who are after this secret. Along the way, he might get help from his usual allies like Alfred. The baby also starts to bond with Alfred, creating some really cute and heartwarming moments in the story.
One interesting plotline could be the kids getting involved in a mystery that Bruce Wayne is trying to solve. They use their own unique skills, like Damian's combat skills or Tim's detective skills, to help their father.
One idea could be that Selina and Bruce have to protect their baby from a new and dangerous villain in Gotham who has a vendetta against the Wayne family. The baby might have some unique abilities that draw the villain's attention.
Bruce's life as Batman would change drastically. He would now have someone depending on him not just as Bruce Wayne but also as Batman. He might have to cut down on his nightly patrols or find more secure ways to keep the baby safe while he's out fighting crime. He would also be more motivated to make Gotham a safer place for his baby to grow up in, so he could focus on taking down the major threats to the city more quickly.
One possibility is that their baby grows up in a world full of duality. Since Bruce is Batman and Selina is Catwoman, the child might struggle with the two different legacies. They could inherit Bruce's sense of justice and Selina's independence and slyness. Maybe the baby has to deal with the public's expectations as the child of two such iconic figures in Gotham.
One interesting Bruce Wayne fanfiction could be about his early days of training before becoming Batman. It might explore how he mastered different fighting styles and detective skills. Another could be a story where Bruce Wayne has to face a new, unknown villain in Gotham while also dealing with his personal life and the secrets he has to keep. There are also fanfictions that focus on alternate universes, like one where Bruce Wayne is a detective in a world without superpowers.
One possible plotline could be about Ginny adjusting to motherhood while still being an active part of the wizarding world. Maybe she has to balance taking care of the baby with her duties as a Quidditch player. Another idea is that the baby has some special magical abilities that are a mystery to Ginny and her family at first, leading to a journey of discovery.
One possible plot could be that Thomas Wayne somehow time - travels to the present where Bruce Wayne is Batman. Thomas could initially be shocked at how Bruce has turned out, but then come to understand his motives. He might offer some old - fashioned wisdom that helps Bruce in his fight against crime.
One popular plotline could be Bruce and Selina going on a secret adventure together outside of Gotham. They might face some unique challenges that test their relationship and skills in a new environment.
One possible plotline could be that Harry Potter somehow finds himself in Gotham City and crosses paths with Bruce Wayne. They could initially be at odds due to their different methods of dealing with problems, but then gradually come to understand and respect each other, leading to a deeper relationship.
One possibility is that Thomas Wayne Batman comes from an alternate timeline. He meets Bruce Wayne in the current timeline and shows him a different way of approaching crime - fighting. Maybe he has more brutal methods, which causes Bruce to question his own more restrained approach. They could then learn from each other and create a new, more effective Batman strategy for Bruce's Gotham.