Some dark wizards or witches who are looking for power might see Harry as a tool or a target. If they think they can harness the power of a hollow - Harry, they would try to capture him. This could lead to many battles and challenges for Harry as he grows up in this strange new form.
In this fanfiction, perhaps Harry's hollow state gives him some unique powers that are different from his normal wizard abilities. For example, he could have enhanced speed or the ability to disappear in a different way. And Voldemort might sense this strange new power and be both intrigued and threatened by it.
In addition to Voldemort, there might be dark wizards who are jealous of Harry's new Hanyou powers. They could team up with some of the anti - Harry Hanyous to create a powerful alliance against him. This would force Harry to make some difficult choices about who to trust and how to fight back.
The Ministry of Magic could be a main antagonist. They have a history of being discriminatory towards werewolves. They might see Harry as a threat now that he's a werewolf and try to lock him up or restrict his movements. For example, they could pass new laws specifically targeting him, like making it illegal for him to attend Hogwarts as a werewolf.
The Ministry of Magic could be a major antagonist. Since necromancy is considered a dark art, they would try to stop Harry at all costs. They might send Aurors after him or try to expel him from Hogwarts.
Another possibility is that the wizarding establishment itself could be the antagonist. They might be afraid of Harry's new form and try to isolate or even eliminate him. After all, a hollow - like Harry could be seen as a threat to the normal order of the wizarding world, and they might not want to take any chances, so they would oppose Harry at every turn.
The main antagonists could be his former friends, like Hermione and Ron. Since Harry is becoming a Dark Lord, they would oppose his new, darker ways. They would try to stop him from causing too much harm to the wizarding world, which would put them in direct conflict with him.
Perhaps the very magic that made Harry a god has a sort of negative side - effect. This could manifest as a shadowy entity within Harry that tries to take over his mind and use his powers for evil. So, in a way, this internal struggle could be the main antagonist in the story.
One potential main antagonist could be a powerful pirate captain who sees Harry as a threat to his control over the seas. Maybe this captain has some dark magic powers as well.
Some jealous students at Hogwarts could also act as antagonists. They might be envious of Harry's newfound allure and use underhanded tactics to try and make his life difficult.
In such a fanfiction, Harry Potter as a Hollow might lose his sense of self and humanity. He could be driven by basic instincts rather than his usual moral compass. This would likely lead to a very different dynamic in the wizarding world as his friends would be faced with either trying to save him or having to oppose him.