Perhaps an ancient evil that was sealed away long ago and has now been released due to some magical mishap. This entity has an instinctive understanding of dark magic that rivals Harry's ultra instinct for good magic. It wants to plunge the wizarding world into eternal darkness and Harry, with his new power, is the only one who can stop it.
It could be a new dark wizard who has also discovered a similar but darker version of the ultra instinct. This wizard wants to use this power to take over the wizarding world and sees Harry as the only obstacle. He would be more unpredictable than Voldemort as his motives and methods are unknown at first.
Voldemort could still be the main antagonist. Even with Harry having the ultra instinct ability, Voldemort's dark magic and his army of Death Eaters would pose a significant threat. His cunning and thirst for power would make him a formidable foe against Harry's new power.
It could be a rogue ninja who has also discovered a dark version of the ultra instinct. This ninja uses his power for evil, like taking over villages and enslaving people. He would be a major threat to Naruto and his friends because his power is so similar yet so different from Naruto's ultra instinct - like powers.
Maybe it could be a new dark wizard or witch who has a unique power that confounds Harry's rational thinking. This antagonist could be someone who uses mind - games and illusions, making it difficult for Harry to use his normal knowledge to fight back. So, he has to rely on his instincts more than ever to figure out their true intentions and defeat them.
Another possibility is a rogue Saiyan from an alternate timeline. This Saiyan could be jealous of Goku's status as the strongest Saiyan in his own timeline. So, they use time travel to come to Goku's timeline and try to defeat him and take over as the dominant Saiyan.
One idea could be Harry discovers he has an ultra instinct ability during a life - threatening battle with Voldemort. This new power allows him to anticipate Voldemort's every move and counter it with ease. It could be a power that is awakened when he is in extreme danger and has to protect his friends and Hogwarts.
It could be a long - lost Uchiha clan member who has a killer instinct amplified by a dark jutsu. This character has a vendetta against the Hidden Leaf for past wrongs done to the Uchiha. They use their Sharingan in very cruel ways, not hesitating to kill even innocent bystanders to achieve their goals.
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.