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.
Some of the older and more conservative members of the Hogwarts staff could also be antagonists. They are used to the old ways of doing things and might not be happy with Harry being in a position of such high power. They could try to block his initiatives, like when he tries to introduce new magical subjects that are more modern and relevant to the current magical world.
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.
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.
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.
Voldemort could still be the main antagonist. He might see Harry's connection to the monsters as a threat to his own power and try to either use it for himself or destroy Harry to prevent him from uniting the monsters against him.
The Death Eaters could be one group of antagonists. They might see Harry's newfound power and position as the King of Valyria as a threat to their plans for wizarding world domination. So they try to stop him from fully realizing his potential in Valyria and bringing back any new magic or alliances that could defeat them.
The pure - blood supremacist wizards who don't want a half - blood like Harry as the king could be the antagonists. They might try to undermine his rule and spread false rumors about him.
One main antagonist could be the existing power - hungry rulers in Essos. They would see Harry Potter as a threat to their rule and try to eliminate him. Another could be dark magic cults in Essos that oppose any form of new leadership or change that Harry might bring.
There could be a magical creature that has been corrupted by dark magic in the North. This creature sees Harry Potter as a threat to its existence and will do everything it can to drive him out or even kill him. It might be very powerful and difficult to defeat, forcing Harry to find new ways to use his magic.