One possible main antagonist could be the headmaster of the killer school. If the school is a place that values strength and ruthlessness, the headmaster might be a tyrant who enjoys seeing students struggle and fail. He or she could be constantly setting up dangerous situations for Harry just to test him.
Perhaps some students who are jealous of Harry's fame could be antagonists. They might try to undermine him during training exercises.
Voldemort would still be a major antagonist. Even in the Marauders era, he was starting to rise to power. Also, the Death Eaters who were following him at that time could be antagonists. They would be causing trouble and trying to spread their pure - blood ideology.
Well, it might be about Harry facing a whole new set of challenges in a so - called 'killer school'. Maybe there are extremely difficult and dangerous courses. For example, he could be learning spells that are not just for defense but for actual combat in a life - or - death situation. And he may encounter new enemies who are far more brutal than the Death Eaters.
There might be an ancient evil entity that has been trapped within the university grounds for centuries. When Harry arrives, it senses his power and tries to use him to break free. This entity could manipulate other students and staff to achieve its goal, making it a very dangerous and complex antagonist.
The main antagonists could be those who don't want Harry to receive a proper Hogwarts education. Maybe some dark wizards who fear that Harry will become too powerful if he goes through the Hogwarts training. So, they plot to keep him away from Hogwarts.
There could be a group of Japanese students who are initially skeptical of Harry but later become his allies. For example, a shy girl who is really good at Japanese magic but lacks confidence. And a tough - looking boy who is actually very kind-hearted and they all form a sort of magical study group together with Harry. There might also be a mischievous Japanese magical creature that befriends Harry and causes some chaos in the school.
One possible antagonist could be the time itself. Since Harry is messing with the natural flow of time, there could be some sort of time - based magic or entity that tries to stop him. It might be a time - keeper or some ancient magic that protects the integrity of the timeline.
Perhaps some of the jealous monster students who don't want Naruto to succeed in school could be antagonists.
The Thalmor could be great antagonists. Their strict control over magic and their anti - human beliefs could put them at odds with Harry. They might see his different form of magic as a threat to their power in Skyrim.
One possible antagonist could be a corrupt wizard in a high - ranking position in the Ministry of Magic's financial department. This wizard might be trying to cover up his misdeeds and would see Harry as a threat to his scheme.