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.
Perhaps the baby is born in a time of great unrest in Gotham. Selina and Bruce have to balance raising their child while also trying to make Gotham a safer place. They could teach the baby values of justice and stealth from a very young age.
Their relationship could be shown as very protective. They are both fiercely protective of their baby and this brings them even closer together. They support each other in parenting decisions and in protecting their family from any threats.
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 good theme could be their reconciliation. For example, after a big fight or misunderstanding, they find their way back to each other. Another theme could be their first encounter from a different perspective, like Selina Kyle seeing Bruce Wayne at a charity event and being intrigued by him.
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.
A great plot twist could be that the wedding is actually part of a much larger plan by a new super - villain in Gotham. This villain wants to use the wedding as a distraction to carry out some major heist or evil scheme. Meanwhile, Bruce and Selina are oblivious to this at first. As they start to notice strange things happening, they have to figure out what's going on while still trying to have their dream wedding.
A plot twist might be that there's some sort of danger related to the pregnancy. For example, an old enemy of Bruce's discovers that Selina is pregnant and tries to use the unborn child as leverage. This could lead to a race against time for Bruce to protect his family.
Perhaps in the fanfiction, Selina Kyle gets pregnant and decides to leave her life of crime behind for the sake of her unborn child. She could struggle with finding a new purpose and fitting into a more normal life. Or, she could be pregnant as a result of some sort of scientific experiment gone wrong in Gotham, and she has to find a way to reverse it while also protecting her baby from those who would use it for their own evil purposes.
One idea could be that Batman and Selina Kyle team up to solve a mystery that involves a new criminal in Gotham who is stealing precious art pieces and using them to fund a more sinister plot. Selina's skills as a cat burglar would be invaluable in tracking down the stolen items.
A popular 'Bruce Wayne Selina Kyle Gotham' fanfic is 'The Cat and the Bat'. In this fanfic, the relationship between Bruce and Selina is shown to be full of tension and chemistry. It's set during a time when Gotham is facing a new and dangerous threat. The story does a great job of showing how they work together to overcome this threat while also dealing with their feelings for each other. There's also 'Selina's Choice in Gotham' which focuses on Selina's perspective. It shows how she views her relationship with Bruce and her place in Gotham. And 'Bruce's Dilemma in Gotham' where Bruce has to make tough decisions that involve Selina and the future of Gotham.
Often, she finds out. But the circumstances and how she reacts can vary depending on the specific comic arc. Sometimes it leads to complex relationships and plot developments.