Perhaps a new dark wizard could be the main antagonist. This wizard could be a former student of Hogwarts who was expelled for dark magic practices. He has since been in hiding, gathering power and creating his own army of dark creatures. He wants to take revenge on Hogwarts and the wizarding world in general, and the player has to stop him before he unleashes chaos everywhere.
Another possibility is an ancient evil force that has been dormant for centuries but is awakened in the game's story. This force could be related to some forgotten magic or an old curse. It starts to corrupt the wizarding world, turning good wizards bad and causing all sorts of disasters. The player has to figure out how to stop this force by exploring ancient ruins, deciphering old spells, and getting help from the wisest wizards in the world.
Voldemort would be a likely main antagonist. He's the most powerful dark wizard in the series. His goal of taking over the wizarding world and eliminating non - pure - blood wizards would create a lot of conflict for the player to face. The player would constantly be in danger of running into his Death Eaters and having to use their magic skills to fend them off.
Well, it could revolve around the fight against Voldemort. The player might start as a new student at Hogwarts, learn spells, and gradually get involved in the battle against the Dark Lord. They could explore places like the Forbidden Forest, find hidden secrets related to the Deathly Hallows, and team up with iconic characters like Harry, Hermione, and Ron.
Perhaps it would be a new character entirely. A powerful wizard who is jealous of Harry's arrival in this new world and sees him as a threat to his own power and status. This wizard could have control over a group of magical beings that are unique to this world and use them to oppose Harry at every turn.
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.
The main antagonist could be a powerful necromancer who combines the dark arts of Skyrim with some elements of Voldemort - like magic. He could be raising an army of the undead and causing chaos across Skyrim, which Harry has to stop.
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.