It could be the werewolf who bit Harry. Maybe the werewolf has some ulterior motives, like using Harry as a pawn in a bigger plan against the wizarding world. Or it could be some pure - blood wizard supremacists who see Harry's new werewolf status as a sign that he is now 'impure' and want to get rid of him.
There's a possibility that the Ministry of Magic could act as the main antagonist. They might be afraid of the implications of a well - known wizard like Harry being a werewolf. So they could try to suppress Harry, perhaps by imposing unfair restrictions on him. This could lead to a conflict between Harry and the Ministry, with Harry fighting for his rights and the Ministry trying to maintain what they see as order in the wizarding world.
The main antagonist might be a dark wizard who has been experimenting on werewolves and wants to use Harry's new condition for his own evil purposes. This dark wizard could be trying to create an army of werewolves under his control and sees Harry as a key part of that plan. Also, it could be an old enemy of Harry's who now sees his werewolf bite as a weakness to be exploited, like Voldemort's remaining followers if the story is set in a post - Voldemort era.
The main antagonist could be an evil werewolf who wants to use Harry's new - found status for his own dark purposes. This werewolf could try to convince Harry to embrace the more savage side of being a werewolf and turn against his friends and the wizarding world. He might also be in league with Voldemort or other dark forces, using Harry as a pawn in a larger, more sinister plan.
It could be a powerful vampire lord who sees the hybrid as an abomination and wants to destroy them. This vampire lord might view the mixing of werewolf and vampire blood as a threat to the purity of the vampire race.
A powerful vampire could be the main antagonist. Vampires and werewolves are often enemies in such fictional settings. This vampire could be after Akakuro's pack's territory or have a personal vendetta against Akakuro himself.
It could be a dark wizard who wants to use the living werewolf hide for some dark magic. Maybe he wants to create an army of werewolves under his control.
It could be a rogue ninja who wants to capture the female werewolf for her unique powers. He might see her as a tool to gain more power in the ninja world.
A power - hungry merman who wants to rule over all the mer - people. He sees Harry as a threat because Harry has some special powers or a claim to the throne that he doesn't want to share.
A new dark wizard who has emerged from the shadows, someone who idolized Voldemort and is trying to pick up where he left off. This antagonist could be using new and more insidious magic that Harry has to figure out how to combat as an Auror.
A power - hungry noble could be the main antagonist. This person might be jealous of the Queen's position and wants to overthrow her, and sees Harry as an obstacle in his way.
Perhaps Voldemort himself, even though he doesn't have the natural ability to shapeshift in the original story. In the fanfiction, he could find a way to mimic shapeshifting, or he could use dark magic to control other shapeshifters to go against Harry. This would make for an even more dangerous and complex conflict as Harry has to face his greatest enemy with this new aspect of his powers.
It could be a villain from Harry's world who is jealous of his new connection with the Starks. Maybe a dark wizard who wants to steal Stark technology to enhance their own dark magic.