Another possibility is that a misunderstanding or a long - held family feud within the Weasley family is the root cause. If there is a Weasley relative who has a hidden grudge against the main Weasley family members and they manipulate Harry into taking revenge on their behalf, then that relative could be considered the main antagonist.
It could also be some external force that is manipulating Harry into believing the Weasleys are his enemies. For example, a dark wizard who wants to create chaos in the wizarding world by turning Harry against his friends. In this case, that dark wizard would be the main antagonist even though Harry is the one carrying out the revenge actions.
Perhaps in this fanfiction, Harry discovers some dark secret about the Weasleys that he believes endangers him or the wizarding world. His revenge could then be to expose this secret in a way that embarrasses the family. He might use his influence and knowledge to make sure the Weasleys are shunned by some in the wizarding community.
It could be Mr. Weasley. Since he's the head of the family, he might be the one driving the idea of taking the Potter fortune to better provide for his large family. He has the authority within the family and might think he's doing what's best in a tough situation.
It could be a rogue Death Eater who survived Voldemort's fall and blames Harry and his sister for the downfall of their cause. This Death Eater might be more cunning and powerful than the others, and he targets Harry's sister specifically because she represents a new threat to their dark plans. He might try to use her as a pawn to draw Harry out and then destroy them both.
Perhaps a long - lost relative of the Potter family who feels they were unjustly excluded from inheriting the manor. This relative could be jealous of Harry's status and the wealth associated with the manor. They might use dark magic to try and claim what they think is rightfully theirs, leading to a battle of magic and loyalty within the Potter family and among Harry's friends.
Another possibility is that the wizarding legal system or a corrupt official within it could be the main antagonist. They might be trying to make things difficult for Harry, or perhaps they are in cahoots with the Weasleys and are trying to cover up the Weasleys' misdeeds. This would add an interesting layer of complexity to the fanfic.
A new dark wizard or witch could be created specifically for this fanfiction. This character could be interested in using the Huntsman's skills for their own evil purposes. If the Huntsman refuses, they become enemies. This antagonist could have powers related to dark forest magic, something that the Huntsman is familiar with but in a more malevolent form.
Voldemort could still be the main antagonist. Even if Demetri has some sort of connection or change in the fanfiction, Voldemort's overarching evil plans would still be a major threat to both Harry and Demetri, especially if Demetri tries to deviate from the Death Eaters' path.
The minotaur itself could be the main antagonist. It's a powerful and dangerous creature, and if it's been set loose in Hogwarts or is guarding something that Harry needs, it would be a major obstacle for him to overcome.
The main antagonist could be a dark wizard who has made the graveyard his lair. He uses the dark energy from the tombstones and the restless spirits there to power his dark magic. Harry stumbles upon this wizard's activities and has to stop him.
A new dark wizard or witch could be the main antagonist. This character, jealous of Harry's fame and power, creates a situation where Harry is framed for a heinous crime. They then ensure his arrest so that they can take over the role of the 'chosen one' in the eyes of the wizarding public and start spreading their own brand of dark magic.