It could be a dark wizard who wants to use the dragons for evil purposes. He might try to control them and make them attack Hogwarts or other wizarding communities.
Maybe there's an ancient dragon that has been corrupted by dark magic. This dragon doesn't like that Harry can talk to the other dragons and tries to stop him from uniting them against the real threats to the wizarding world. It sees Harry as a threat to its own power and plans to destroy him.
There could be a group of dragon hunters in the fanfiction. They don't want Harry to communicate with the dragons because they fear that it will lead to the dragons becoming more powerful and harder to hunt. They are willing to do anything to stop Harry, even if it means allying with dark wizards. They see the dragons as nothing more than trophies and a source of wealth, and Harry's interference with their plans makes them the main antagonists.
Well, it could be that the dragons are in danger due to some dark magic. Harry talks to them to figure out a way to save them. In the process, he learns more about the magical world of dragons and their history. He might even find out that the dragons have a connection to the founders of Hogwarts.
It could be a rogue ninja who has the power to corrupt the trees. This ninja wants to use the power of the forest for evil, like creating a dark chakra source from the trees' life force. Naruto has to talk to the trees to figure out how to stop this ninja.
Another possibility is the Volturi. Their strict rules and power - hungry nature could be a threat in the background. Jasper might be warning Bella about the Volturi's watchful eyes and how they view her relationship with the Cullens. He could be explaining how their actions or decisions could impact not only Bella but the entire Cullen family.
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.
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.