It could be a long - lost relative of Voldemort. Maybe a distant cousin or something who has the same thirst for power and has decided to target Harry and his friends to gain more influence in the wizarding world.
It could involve Harry Potter taking a stand against a dark wizarding family that has wronged his loved ones. Maybe they've targeted his friends or family, and he decides that a blood feud is the only way to deal with their ongoing threats and attacks.
Obviously, Harry Potter himself could be a central character. He might be drawn into the blood feud because of his reputation and his desire to maintain peace in the wizarding world. He could try to mediate between the feuding families.
It could be Voldemort. He might have cast a new spell on Harry that makes him cough blood as part of his ongoing attempts to defeat Harry.
Hermione and Ron would surely be his top allies. Hermione with her vast knowledge of magic and Ron with his loyalty and combat skills. They've been with Harry through thick and thin. Also, the members of the Order of the Phoenix like Sirius Black (if he's still alive in the fanfiction), Lupin, and Moody could be great allies. They have experience and are very powerful wizards.
Obviously, a Potter and a Weasley would be the main characters. It could be Albus Potter and Rose Weasley, for instance. They would be the ones most directly affected by the feud between their families.
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.