It could be a former Death Eater who escaped justice and has been biding his time. Now that Harry has a son, he sees an opportunity to strike at the heart of the wizarding world again. This antagonist might have a personal vendetta against Harry and wants to make his son suffer as a way to get back at him. He could be using dark magic to create all sorts of problems in Hogwarts and the surrounding areas.
A new dark wizard could be the main antagonist. Maybe someone who was inspired by Voldemort's ideology but has new and more devious ways of causing chaos in the wizarding world. This antagonist could target Harry Potter's son to prove himself as a more powerful and feared dark wizard.
A powerful dark wizard who has a grudge against those with fire - related magic could be the main antagonist. He might see Harry's newfound 'Son of Fire' powers as a threat to his own plans for domination.
Voldemort could still be the main antagonist. Even though Harry has the blood of Darth Vader, Voldemort's goal of taking over the wizarding world remains the same. Harry would have to use his combined powers from both his wizard and Vader heritage to defeat him.
A new dark wizard or witch could be introduced as the antagonist. This character could be even more powerful and cruel than Voldemort. They could have a special power that allows them to break Harry's spirit, perhaps by constantly torturing his mind or by causing harm to those he loves in ways that are even more insidious than Voldemort's methods.
Perhaps a long - lost relative of Voldemort who has been in hiding all these years. This relative has discovered a way to gain power through Harry's kid. They might start by infiltrating Hogwarts under a false identity and slowly manipulate events to get closer to the kid. Their ultimate goal could be to sacrifice the kid in a dark ritual to bring back the dark days of Voldemort's reign.
Voldemort could still be the main antagonist. His dark magic would be in direct contrast to Harry's angel inheritance. He would see Harry as an even greater threat now and would do everything to stop Harry from fully realizing his new powers.
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.