It could still be Voldemort, but in a different form. Maybe he has a new power or a different plan this time. Since it's a new chance, he might have found a way to come back even stronger and more devious than before.
It could still be Voldemort. Even though Harry has a second chance, Voldemort is still a major threat in the wizarding world. His presence and his Death Eaters would continue to be the main source of danger, and Harry would need to use his second chance wisely to defeat him once and for all.
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.
There could be a dark spirit or entity that has been awakened by Harry's arrival in New Orleans. This entity was once sealed away but Harry's magic has somehow loosened its bonds. It then starts causing chaos in the city, attacking both muggles and wizards alike. Harry has to figure out how to stop this entity, but first he has to deal with the fact that the local magical community blames him for the entity's awakening.
Perhaps it's a corrupted Hogwarts professor. In this fanfiction, a teacher could have been influenced by the dark side and decides to oppose Harry's new path. They could be using their position at Hogwarts to spread dark magic teachings or to manipulate students against Harry. They would be a more insidious antagonist as they are operating from within the very institution that is supposed to be a safe haven for Harry.
It could be a new dark wizard who has emerged in Harry's new life. This wizard might be jealous of Harry's fame and want to take him down.
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.