It could be a dark wizard who has a vendetta against the Weasley family. Since the twins are part of the Weasleys, this dark wizard targets them. Maybe he blames the Weasleys for something that happened in the past, and the twins, being the more adventurous and visible members of the family, become the main targets.
Perhaps a corrupt Ministry of Magic official could be the main antagonist. In this different life fanfiction, the official might be trying to pass laws that would oppress the twins' creativity and business with their joke shop. The twins would then have to fight against this unjust system, using their wits and magic to expose the corruption within the Ministry.
In this fanfiction, it's possible that a powerful and jealous rival at Hogwarts could be the main antagonist. This rival could be from Slytherin, envious of the twins' popularity and their natural talent for magic. The rival might constantly try to undermine the twins, from sabotaging their pranks to trying to get them expelled from Hogwarts.
Perhaps it would be a new character entirely. A powerful wizard who is jealous of Harry's arrival in this new world and sees him as a threat to his own power and status. This wizard could have control over a group of magical beings that are unique to this world and use them to oppose Harry at every turn.
In a 'Harry Potter Twins A Different Life' fanfiction, the twins could have a different life path due to a time - travel incident. Imagine they accidentally travel back in time and meet their parents when they were young. This encounter could change their understanding of their family history. They might also influence the past in some small ways, which in turn alters their present. For instance, they could prevent a small accident that would have otherwise affected their family's financial situation, leading to a more prosperous Weasley family in their original time.
Maybe a group of pure - blood supremacists who didn't like the changes that Harry brought to the wizarding world after the war. They believe that the old ways of pure - blood dominance should be restored and will stop at nothing to oppose Harry's new beginning, which includes more equality among all wizards and witches.
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.