Maybe there's a group of hunters who are aware of her existence. They think that any offspring of a demon like Crowley is pure evil and must be destroyed. They would hunt her down relentlessly, not realizing that she might not be as bad as they assume.
A powerful demon who wants to take over Crowley's territory might target his daughter. By getting rid of her, they believe they can weaken Crowley and then take what he has. This demon could be very cunning and use all sorts of tricks to get to her.
It could be a powerful rogue ninja who has a vendetta against Kakashi and sees his daughter as a way to get revenge. This antagonist might try to use her as a bait to lure Kakashi or Naruto into a trap.
It could be a rogue scientist who wants to study Jame's daughter's bionic powers for his own evil purposes. He might be someone who was once involved in the bionic program but had his own unethical ideas, so he was kicked out.
Maybe an organization that wants to use her unique abilities. If she has inherited some special power from Kakashi or has her own distinct skills, an evil organization could try to capture her and force her to work for them. They would be the main obstacle she has to face in the story.
The mother's new partner. He doesn't want the forgotten daughter to be part of the family as he fears she will disrupt the new family dynamic he has been trying to build. He might use various means to keep her away from the family.
Another option is a power - hungry wizard from within the Ministry of Magic. They might see Harry's daughter as a tool to gain more power. They could try to manipulate her or create situations where she is framed for things she didn't do, all in an attempt to control her and use her connection to Harry for their own political gain.
Perhaps a group of anti - technology extremists could be the main antagonists in an Ultron fanfic. They see Ultron as the symbol of all that is wrong with technology and are determined to take him down, no matter the cost. They might use their own advanced, but more rudimentary, technology to fight against Ultron. This would create an interesting contrast between Ultron's high - tech abilities and their more basic, yet effective, methods.
A powerful wizard or sorcerer who wants to use Goku's daughter's pure Saiyan energy for some dark magic. He could be creating chaos on Earth, and she has to stop him before he drains all her energy. This antagonist would be a departure from the typical physical - strength - based villains in Dragon Ball, adding a new dimension to the fanfiction.
It could be a rogue Wakandan scientist who is jealous of the daughter's unique abilities. He might see her as a threat to his own plans of taking over Wakanda's technology and using it for his own evil purposes. So, he tries to undermine her at every turn and stop her from fulfilling her destiny in Wakanda.
A group of rogue demigods could be the main antagonists. They might be jealous of the attention the daughter gets because of her parents. So, they plot to make her life difficult and try to drive a wedge between her and the other demigods.
It could be a powerful vampire from a different coven. Maybe one who sees the daughter of war as a threat to their power or territory in the Twilight world.