A rogue slayer who has turned to the dark side. She might envy Buffy's daughter's connection to Buffy and the legacy she represents. This rogue slayer could be using her powers for evil, like causing chaos in the human world and trying to eliminate any competition, which includes Buffy's daughter.
The Joker would be a classic choice. His chaotic and unpredictable nature would pose a great threat to Batman's daughter. He might try to corrupt her or use her as a pawn in his evil schemes against Batman.
It could be some business rivals who see the daughter as a threat to their plans. They might try to undermine her or use her to get to Stark Industries.
A really interesting antagonist could be a mysterious figure from a hidden organization. This person has been secretly observing Naruto's family for a long time. They have a master plan that involves using Naruto's daughter as a key component. Maybe they want to use her unique chakra or abilities for some dark purpose. They would be constantly manipulating events behind the scenes, setting up traps and challenges for her, all while remaining in the shadows, which would make them a very menacing and unpredictable foe.
I'm not sure which specific Buffy adult daughter fanfiction is the best as there are so many out there. However, you can start by looking at popular fanfiction platforms. Some fanfics might focus on the family dynamics between Buffy and her daughter, like how they communicate and resolve conflicts. Others could be more action - oriented, with the daughter following in Buffy's footsteps as a slayer. You should also check the reviews and ratings of different fanfictions to find the ones that suit your taste.
The main antagonists could be those who envy her wisdom. They might be jealous of the attention she gets or the power that her wisdom gives her. So, they try to undermine her at every turn, spreading false rumors about her or trying to make her look foolish in front of important people.
It could be the old enemies of Jack Sparrow. They might see his daughter as a way to get revenge on him. For example, those he has double - crossed in the past.
The descendants of the Death Eaters would make great antagonists. They might still hold a grudge against Sirius Black for his actions against Voldemort and his followers. They could target his daughter to seek revenge or to prevent her from uncovering any secrets that Sirius might have known about them.
The Penguin could be a main antagonist. He might see the daughter as a new threat to his criminal operations in Gotham. Since she has the combined skills of Batman and Catwoman, she could easily disrupt his illegal business, like his smuggling operations.
It could be a new group of rogue ninjas who oppose the ideals of the Hidden Leaf Village. They might target Kakashi's daughter because of her connection to Kakashi and Naruto, seeing her as a symbol of the village's new generation of strength.
Common themes include the struggle between normal life and the supernatural. The daughter may want a normal life but is constantly drawn into the battles like Buffy was. Another is the theme of mentorship. Buffy may mentor her daughter in slaying, or the daughter may find other mentors. There's also the theme of redemption. If the daughter makes mistakes or has a dark side, she may seek redemption, much like some of the characters in the original Buffy series did. These themes help to create interesting and engaging stories in the fanfiction world.