There could also be some characters from the upper echelons of the society that benefits from slavery. They might not be directly involved in the day - to - day management of slaves like the slave owners, but they support the system through laws and policies. They would oppose Harry's attempts to gain freedom as it would disrupt the status quo that they profit from.
Perhaps there could be some overseers or enforcers who are especially brutal towards the slaves. They might take pleasure in punishing the slaves and would be a major obstacle for Harry. These characters could be loyal to the slave - owning system and do everything to maintain it, seeing any sign of resistance from Harry as a threat.
The old - fashioned and conservative gods who don't like the idea of a new, young god like Percy shaking up their established order could be antagonists. They might see him as a threat to their power and try to undermine him.
Another vampire, an older and more powerful one, could be an antagonist. This vampire might see Percy as a threat to his power or territory. Maybe he wants to either kill Percy or make him submit to his rule.
Another possible antagonist could be some of the more power - hungry Titans. They might see Percy as a rival or someone who could disrupt their plans. They could try to manipulate him or turn him to their side, and if he refuses, they would become his enemies. Also, there could be some ancient monsters that sense Percy's new power and want to destroy him before he becomes too powerful.
The Titans could be great antagonists. They might see Percy as a threat to their plans of overthrowing the gods again. Since Percy has chaos powers, they could fear that he could disrupt their carefully laid - out schemes. Another possibility is some of the more traditional enemies of the demigods, like the monsters. They might be drawn to Percy's chaotic energy and try to either consume it or destroy him to prevent him from becoming too powerful.
The Ministry of Magic could be the antagonists. Since Harry is in a gang, they might see him as a threat to the order of the wizarding world. They could send Aurors after him and his gang mates, trying to arrest them and break up the gang.
Another possible antagonist could be a powerful ghoul organization that wants to use Kaneki for their own purposes. They might try to capture him and experiment on him to figure out how to create more half - humans or use his powers for their own gain. They don't care about Kaneki's well - being and are only interested in what he can do for them.
The main antagonists could be the villagers who fear Naruto's mini Kyuubi form and try to drive him out of the village. Their prejudice and fear act as a major obstacle for Naruto, who just wants to be accepted.
The Thalmor could be great antagonists. Their strict control over magic and their anti - human beliefs could put them at odds with Harry. They might see his different form of magic as a threat to their power in Skyrim.
Some of the more traditional pure - blood families in the wizarding world could be antagonists. They might not approve of Harry, a half - blood, being involved with Pansy, and they could try to stop the pregnancy or break up the couple for the sake of 'pure - blood' ideals.
Voldemort or his followers could still be antagonists. They might not like the idea of Harry being under Euryale's protection as it could make him more powerful. Another possibility could be some creatures from Euryale's world who are against her taking in an outsider like Harry.