Another possibility is a vengeful former Death Eater's apprentice. This person blames Harry for the downfall of their master and the loss of the Death Eater regime. Seeing Harry as a weakened target because of his blindness, they plot to exact revenge on him. They might use dark magic to create illusions and traps specifically designed to disorient Harry, taking advantage of his lack of sight.
A rogue ninja from a formerly warring village could be the main antagonist. This ninja might hold a grudge against the new peace and wants to disrupt it. He could be powerful and have his own group of followers who are also discontent with the current situation.
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.
The plot could revolve around Harry's rehabilitation. Ginny could be by his side, training him to use his other senses. Maybe they work with magical healers who try different spells to restore his sight, but it doesn't work. So they focus on making his life as normal as possible. Ginny would teach Harry how to move around Hogwarts without bumping into things, and they'd have a lot of cute, clumsy moments in the process.
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.