There could be a group of rogue hitmen in the wizarding world who don't like the idea of a female Harry Potter encroaching on their territory. They see her as an outsider and a threat to their way of life. This group might have their own code of conduct that they believe fem Harry Potter is violating. They could be very skilled in both magical and non - magical combat techniques, and they would constantly be trying to ambush or outwit fem Harry Potter in order to drive her out of the hitman business.
It could be a powerful dark wizard who is also involved in the hitman business. This wizard might see fem Harry Potter as a threat to his operations.
It could be a dark wizard who wants to take over the farm for its magical properties. Maybe there's a hidden well of ancient magic there that he desires.
Maybe a new character that is created specifically for this fanfiction. This character could be jealous of the attention fem Bowser is getting from both the wizarding world and Harry Potter's group. They could start causing trouble and trying to turn everyone against fem Bowser. This new antagonist could also have some connection to Bowser's original world, perhaps an enemy from there who has followed her to the Harry Potter universe.
Well, perhaps the female Harry Potter uses her magic skills in the world of being a hitman. She could be using spells to outwit her targets or to make her escapes. Maybe she has a special wand that she uses as a weapon in a more non - traditional hitman way.
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.