The Chaos Gods themselves could be the main antagonists. Their very nature is to create chaos and disorder, which would be directly opposed to the order and stability that Hogwarts and the wizarding world try to maintain.
A being created by the Chaos Gods, a sort of chaos - infused monster. It could be a combination of different magical creatures, with abilities that are constantly changing and unpredictable. This monster would be sent to disrupt Hogwarts and the surrounding areas, causing destruction and mayhem as it goes about its mission for the Chaos Gods.
Zeus could be one. He is a powerful god in Greek mythology and might be interested in Harry's story. Another could be Odin from Norse mythology, known for his wisdom. And perhaps Isis from Egyptian mythology, as she has powers related to magic and protection.
It could be Zeus. He is a powerful god and might see Harry as a worthy candidate for training. He could teach Harry about leadership and power control.
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.
In a 'Harry Potter Chaos Gods' fanfiction, a cool plot could be that the Chaos Gods create their own version of the Triwizard Tournament. But instead of the normal challenges, they design tasks that are full of their own brand of chaos. The champions not only have to face dangerous magical creatures but also deal with the unpredictable and often maddening effects of the Chaos Gods' interference.
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.