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.
The Death Eaters who still hold onto the old pure - blood supremacist views could be main antagonists. They might see the Heir of Arcturus Black as a threat to their ideology, especially if the Heir is trying to change the Black family's ways.
Hermione Granger could also be a possible Hogwarts heir. She is extremely intelligent and has a thirst for knowledge about Hogwarts' history and magic. Her Muggle - born status might seem to be a contradiction, but it could also be part of what makes her unique as the heir. She could bring new perspectives and use her vast knowledge to unlock secrets about the heir's role and the future of Hogwarts.
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.
In the fanfiction world, Harry might be the heir to Hogwarts if there was some sort of magical pact made generations ago. For example, perhaps one of his ancestors had a special role in the creation or protection of Hogwarts, and this role was passed down through the family. It could also be that his unique magical abilities, like his ability to resist Voldemort's attacks from a young age, are signs of a deeper connection to Hogwarts itself. This connection could be what designates him as the heir in the fanfiction's plot.
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.
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.
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.
The Death Eaters could still be antagonists. Even if Harry is the heir of the four founders, Voldemort and his followers would not want him to gain more power. They would try to undermine Harry's new - found status and use it to their advantage, perhaps by spreading false rumors about him among the wizarding community.
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.
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.