Perhaps some old enemies from Harry's past who were thought to be defeated but have now resurfaced. They target Lemon because they know how much she means to Harry. For example, a follower of Voldemort who escaped justice and now wants revenge on Harry by harming his daughter. This antagonist would bring back the sense of danger and the need for Harry to protect his family like he did in his own youth. Another possible antagonist could be a strict Hogwarts professor who has a grudge against Harry and takes it out on Lemon, making her school life difficult and causing a rift between father and daughter as they try to deal with this unfair treatment.
A new dark force could emerge in the wizarding world that specifically targets the Potter family. This entity might see the Potter bloodline as a threat to its plans for domination. Lemon, being the new generation of the Potter family, becomes a prime target. The antagonist could be a mysterious and powerful dark wizard or a malevolent magical being that has been awakened after a long slumber. Harry and Lemon would then have to team up with their friends and allies to face this new threat.
It could be a group of dark wizards who are jealous of Harry's status and see Lemon as a way to get to him. They might try to kidnap her or manipulate her to get Harry to do their bidding.
One potential antagonist could be some pure - blood supremacists who don't like the idea of Harry Potter having a half - blood or Muggle - born daughter. They might try to cause trouble for Lemon at Hogwarts or in the wizarding world in general. Another could be a dark wizard who sees Lemon as a threat for some reason, perhaps because of her special abilities or her connection to Harry. They would try to harm her to get to Harry.
One possible plot could be that Harry's daughter Lemon discovers she has a unique magical ability that Harry has to help her control. Maybe she can communicate with magical creatures in a way no one else can, and they go on adventures together to learn more about this gift.
Voldemort or his remaining Death Eaters could be antagonists. They might see her as a threat due to her unique combination of powers. Also, there could be some anti - mutant groups in the wizarding world who oppose her existence.
It could be some of the pure - blood supremacists. They might see her as an abomination because of her connection to Hecate, who is not part of the traditional wizarding families. They would try to undermine her at every turn and make her life difficult at Hogwarts.
It could be Draco Malfoy. He might not like the idea of Harry getting a second chance at the OWLS and tries to sabotage him. Maybe he spreads rumors about Harry not being worthy of a retake.
It could be a new dark wizard who wants to use Harry's daughter to get revenge on Harry. This wizard might be jealous of Harry's fame and power and think that by harming his daughter, he can destroy Harry's happiness.
The main antagonists could be the scientists or the people in charge of the Weapon X program. They would see Harry as an experiment or a threat to their plans.
There could be some old - fashioned professors at Hogwarts who are against Harry's unorthodox ways of making his mark. They believe in the old - established ways of teaching and practicing magic and see Harry as a troublemaker who is disrupting the order of things at the school.
One possible plot could be that Lemon discovers a dark secret about the wizarding world that even Harry doesn't know. As they try to uncover the truth, they face opposition from some powerful wizards who want to keep the secret buried. Harry uses his skills and connections, and together with Lemon's fresh perspective and new - found magic, they fight against those who would stop them. Along the journey, their relationship as father and daughter grows stronger as they rely on each other for support and protection.
Voldemort's remaining followers could be antagonists. They might target Harry's little sister thinking they can use her to get to Harry. They could try to kidnap her or use dark magic on her.