It could be another powerful wizard who wants the grimoire for himself. He might be jealous of Harry's connection to it and will stop at nothing to get it. This wizard could have a dark past and be using the grimoire as a means to gain ultimate power over the wizarding world.
There's also the possibility that the main antagonist is an ancient evil that has been trapped for centuries. The grimoire's use by Harry somehow awakens this evil, and it now targets Harry specifically. This ancient evil could have powers far beyond what Harry has ever faced, and it might be able to possess other wizards and creatures to do its bidding, making it a very difficult enemy to fight.
Maybe it's a group of dark wizards. They see the power that the grimoire could give Harry and decide to form an alliance to take it from him. They could be from a different magical region where dark magic is more prevalent, and they believe that the grimoire belongs to them by right of their more 'traditional' view of magic.
In this fanfiction, the main antagonist could be an ancient evil that has been trapped within the grimoire. Once Harry finds the grimoire, this evil starts to gain power and tries to use Harry to break free completely and spread chaos in the wizarding world.
In the fanfiction, the grimoire might be the key to unlocking Harry's true potential. He could have been holding back in his magic all this time, and the grimoire shows him new ways of thinking about and using magic. It might also introduce him to a new group of magical creatures that he has to team up with to face a common enemy. This would not only expand the magical world in the fanfiction but also give Harry a whole new set of allies and challenges.
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.
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.
It could be Malfoy. He might make fun of Harry for wearing a dress and try to use it to humiliate him in front of the whole school, which would then lead to a conflict between them that goes beyond the usual rivalry.
Perhaps a new dark wizard or witch who has infiltrated the Ministry of Magic and made sure Harry was sent to Azkaban. This antagonist could be working towards a bigger plan to take over the wizarding world and sees Harry as a major threat even when he's locked up.