One interesting plot could be the daughter's struggle to live up to Batman's legacy while also forging her own identity. She might face villains who target her because of her relation to Batman.
One interesting plot could be the sons teaming up to solve a mystery that Batman himself has trouble with. They might use their unique skillsets, which are different from Batman's, to approach the case from a new angle.
Another plot could involve her struggle to fit in with normal society while having this extraordinary phantom - related background. She may face discrimination or misunderstanding from others, but in the end, she manages to find a group of people who accept her for who she is.
One interesting plot could be her discovery of her father's past. Maybe she stumbles upon some old artifacts or memories that lead her to understand the sacrifices he made. For example, she might find his old Death Eater's mark and then try to piece together what it means for her family's history.
One interesting plot could be Bella growing up in the Cullen family with Carlisle as her father. She might inherit some special vampire traits early on and have to deal with the unique challenges that come with it, like hiding her abilities from humans. Another plot could involve her relationship with the other Cullens, perhaps having a different dynamic with Edward as her 'brother' which changes the whole Twilight love story.
In some fanfictions, her plot could revolve around time - travel. She might accidentally travel back in time and meet her father when he was a student. This would give her a chance to see a different side of him and understand his choices better. She could also try to change some events in the past to make a better future. However, this would bring a lot of consequences and she would have to deal with the butterfly effect in the wizarding world.
In some fanfictions, their daughter might face a great threat to the wizarding world. Maybe a new dark wizard emerges, and she has to step up and use her combined heritage from Harry and Hermione, as well as her own skills and courage, to defeat the enemy. She might struggle with the pressure at first but eventually grow into a great hero.
One interesting plot could be that Batman and Catwoman team up to save Gotham from a new and powerful villain, and during their adventures, their feelings for each other deepen. They start to see beyond their usual cat - and - mouse game.
A cool plot could be a crossover where the Saints and Batman team up against a common enemy. This enemy could be a super - villain who has somehow made their way into the Saints Row world and is causing mayhem on a scale that neither group can handle alone. They would have to combine their skills, the Saints' street - smart and unorthodox tactics with Batman's combat and detective expertise.
A plot could involve the reader having a special ability that both Batman and the Joker are interested in. Batman wants to use it for the good of Gotham, while the Joker wants to corrupt it for his own amusement. The reader has to navigate between these two very different forces, and in the process, discovers more about themselves and the nature of good and evil in Gotham.
Perhaps in the fanfiction, Batman is initially hesitant about his feelings for Supergirl because of their different powers and backgrounds. Supergirl, on the other hand, is more open and tries to break through Batman's tough exterior. Their romance could be a slow - burn one, with Supergirl using her kindness and strength to show Batman that they can be together, and they eventually fall in love while saving the world from a powerful alien threat.