It could be some dark wizards who are trying to take over the wizarding world outside of Hogwarts' influence. Since Hogwarts is often a place that trains wizards to fight against dark magic, without it, these dark wizards might have more freedom to operate.
Another possibility is that a group of radical pure - blood wizards who oppose any form of non - traditional magic education could be the antagonists. They might see Harry as a symbol of a new way of learning magic that they don't approve of, and thus try to stop him from spreading his unorthodox knowledge and power.
The main antagonists could be those who don't want Harry to receive a proper Hogwarts education. Maybe some dark wizards who fear that Harry will become too powerful if he goes through the Hogwarts training. So, they plot to keep him away from Hogwarts.
The plot could develop with Harry being mentored by a solitary wizard or witch who doesn't believe in the Hogwarts system. They could teach him magic in a more unorthodox way. For example, instead of formal classes, he could learn magic through real - life experiences and solving problems in the wizarding community outside of Hogwarts. This would give him a different perspective on magic compared to the traditional Hogwarts - educated wizards.
The pure - blood supremacist families could be the main antagonists. They might see Harry as a threat to their traditional ways of running Hogwarts. They would oppose his new rules and reforms that aim for a more inclusive and equal school. For example, they could try to undermine his authority by spreading false rumors about him among the students and their families.
Voldemort and his Death Eaters could still be the main antagonists. Since Harry is now seen as a prince, they might see him as an even greater threat to their dark plans and try to eliminate him.
One possibility could be a group of dark wizards who are jealous of Hogwarts' power and reputation. They want to bring it down and Harry, as the champion, stands in their way. Another could be a former Hogwarts student who turned to the dark side and has a vendetta against the school for some reason. They target Harry specifically because he represents everything good about Hogwarts.
It could be a group of former Death Eaters who escaped justice and are now plotting to take over Hogwarts again. They see Harry Potter training the students as a threat to their plans.
The pure - blood supremacists could be antagonists. They might not like the idea of Harry, a half - blood, inheriting Hogwarts and try to undermine his authority.
Another possible antagonist could be a power - hungry pure - blood family. They might see Harry as an impostor as the heir, believing that only a pure - blood should hold such a title. They would try to undermine Harry at every turn, spreading rumors and creating obstacles for him both in school and in the wizarding world.
Often, it could be dark wizards or witches who are envious of Hermione's status as the heir. They might want to take over Hogwarts themselves or steal the power that comes with being the heir. For example, a powerful dark wizard who has been in hiding for years might emerge to challenge Hermione.
One possible plot could be that Harry Potter discovers a different magical school. Maybe he stumbles upon a hidden school in the Muggle world that has been secretly teaching magic. He could form new friendships and face different types of magical challenges there. It would be a completely new magical experience for him, away from the familiar Hogwarts environment.