There might be a character who is secretly from the wizarding world but has turned to the dark side. This character could be after Harry in the Ouran setting, trying to use the chaos of the new environment to their advantage to get to Harry without being noticed by the Hogwarts professors.
Well, Harry could be really confused at first. Ouran is a very different world from Hogwarts. He might be shocked by the opulence of Ouran Academy and the strange club system there. Maybe he'd try to find a way to fit in while still being true to his wizard self.
It could be a new dark wizard that we haven't seen in the original series. This wizard might be jealous of Harry's fame and power and wants to eliminate him by making him disappear.
A jealous classmate could be the main antagonist. This person might be envious of Harry's popularity or the strange things that seem to always work out for him. They could start spreading rumors or trying to trip him up in his studies.
It could be a new dark wizard who has some personal vendetta against Harry. Maybe this wizard has a grudge because Harry defeated Voldemort and he wants to prove he's more powerful, which enrages Harry and causes him to go on a rampage.
It could be a new dark wizard who has the power to manipulate minds. This wizard might target Harry specifically, driving him insane as part of a larger plan to take over the wizarding world. Since Harry is seen as a hero, if he goes insane, it would create chaos and make it easier for the antagonist to achieve his goals.
The main antagonist could be a new character, a corrupt official in the Ministry of Magic who wants to get rid of Harry because he's seen as a threat to their power. This person could manipulate the legal system to send Harry to jail.
The main antagonist could be the Devil himself. He might see Harry as an intruder in his domain and try to keep him there forever. He could set up all kinds of traps and challenges for Harry to overcome.
It could be a new dark wizard who has been lurking in the shadows. Maybe someone who was once a student at Hogwarts but turned to the dark side.
There could be a dark entity that lurks within the Durmstrang castle itself. It might have been awakened by Harry's arrival or has some connection to the school's past. This entity could manipulate students and teachers alike to create chaos and oppose Harry.
It could be a corrupted prison guard. They might be taking advantage of the chaos in Azkaban to carry out their own evil plans, like stealing magic from the prisoners. Harry would have to stop them while also dealing with his own situation in the prison.