Some of the pure - blood supremacist families might also be antagonists. They already look down on those with 'impure' blood or those who are different. Harry as a werewolf would be an easy target for their hatred. They could try to use his new condition to turn public opinion against him and undermine his status as the 'Chosen One'. They might even plot to have him killed, thinking that a werewolf 'Chosen One' is an abomination.
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.
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.
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.
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.
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.
One possible plot could be that Harry is bitten by a werewolf during one of his adventures outside of Hogwarts. He then has to struggle with his new identity as a werewolf. He might face discrimination from some in the wizarding world who fear werewolves. He could also try to find a way to control his transformations, perhaps with the help of his friends like Hermione who would research potions or spells to ease his condition.
The Order of the Phoenix would be likely antagonists. Since Harry has turned to the dark side, they would see him as a threat to the peace and safety of the wizarding world. Dumbledore, if he were still alive in the fanfiction, would also oppose Harry. He would try to use his wisdom and power to bring Harry back to the light. Another antagonist could be Neville Longbottom. Neville, who has always been a symbol of resistance against the dark side, would not stand for Harry becoming a dark lord and would fight against him to protect Hogwarts and the values it stands for.
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.
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.
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.
Scarecrow could also be a main antagonist. His fear - inducing toxins could be a real problem for Harry. Harry would have to use his magic to counter the effects of the fear toxin and also figure out a way to stop Scarecrow from spreading it all over Gotham.