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.
It could be Voldemort. Since he is the main bad guy in the Harry Potter series, it would make sense for him to be the antagonist. He might see the daughter of Thor as a threat to his plans of taking over the wizarding world.
It could be a power - hungry Death Eater who didn't like how Voldemort was running things and decides to take over after his defeat. This character might be even more cruel and calculating than Voldemort, using dark magic in new and terrifying ways.
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.
Perhaps a dark wizard who is jealous of Harry's new power. This wizard could be someone who has long been in the shadows, waiting for the right moment to strike. He might see Harry as a threat to his own plans of taking over the wizarding world.
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.
A powerful and ancient Kryptonian entity that was sealed away long ago could be the main antagonist. It breaks free due to some events and wants to reshape Krypton according to its own dark desires. The reborn character, with their new - found powers and different perspective, stands in its way. Also, there could be a group of Kryptonian zealots who oppose any change in the traditional Kryptonian ways. They see the reborn character as a threat to their way of life and try to get rid of them.
Lex Luthor would be a classic choice. He could be jealous of the protagonist who is reborn as Superman and see him as a threat to his own power and influence. Luthor might try to use his intellect to create new weapons or strategies to defeat Superman.
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.
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.