The other students at Hogwarts might become the antagonists. They have known Harry as a hero, and when he turns to Voldemort, they would be horrified. They could form a resistance within the school to oppose Harry and Voldemort, making them the main antagonists in this scenario.
The main antagonist could be Dumbledore. Since Harry has joined Voldemort, Dumbledore would see Harry as a threat to the good of the wizarding world. Dumbledore would try to bring Harry back to the right side, and they would end up in conflict.
It could be a new dark wizard who has been secretly building his power. Maybe this wizard uses the allure of the cane and the idea of a new kind of dark magic to draw Harry in. He might be a former follower of Voldemort who has his own plans for domination and sees Harry as a useful tool.
Snape could play a crucial role. Given his past with both Voldemort and his role at Hogwarts, he might be torn between his loyalty to Dumbledore's cause and his complex relationship with Harry. He could be the one who has to make a difficult decision when Harry starts to side with Voldemort. Additionally, Draco Malfoy could be involved. He might see Harry's change as an opportunity to gain more power within Voldemort's ranks, or he could be jealous of Harry's new - found position with the Dark Lord.
It could be a power - hungry Death Eater who didn't like how Voldemort was running things and decides to take over after his defeat. This character might be even more cruel and calculating than Voldemort, using dark magic in new and terrifying ways.
Perhaps a new dark wizard or witch who sees Harry as a threat. Since Harry has the power of Voldemort version 2, this new antagonist might fear that Harry will stop their plans for world domination. They could be someone who was in the shadows during Voldemort's time and now wants to take advantage of the chaos that Harry's transformation brings. They would try to use Harry's new identity against him and turn the wizarding world against him even more.
In a 'harry joins voldemort fanfiction', perhaps Harry discovers a dark secret about the wizarding world that makes him think Voldemort was right all along. It could be something about the Ministry of Magic being corrupt and Voldemort's way of ruling, although extreme, is the only solution. So he joins Voldemort to 'fix' the wizarding world.
A group of rogue Death Eaters could be the main antagonists. They think that Voldemort has gone soft by teaming up with Harry. So they break away and form their own more extreme and violent group. They start attacking both the wizarding world and anyone associated with Harry and Voldemort's new alliance. Harry and Voldemort must then fight against their former allies who have become a greater threat than ever.
It could be a dark fairy who is jealous of the Fairy King's alliance with the wizarding world. This dark fairy wants to destroy the connection and take over both the fairy realm and the wizarding world.
It could be a rogue Death Eater who decides to carry on Voldemort's legacy in a more extreme way. This Death Eater might have some new and terrifying dark magic up their sleeve.
Hermione could be the one. Maybe she gets jealous of Harry's fame and power, and starts spreading lies about him, which leads to his betrayal. As a result, Harry feels so hurt that he turns to Voldemort.
Hermione could be the main character. She is extremely intelligent and powerful in her own right. She might use her vast knowledge of magic to find a solution that doesn't involve Harry fighting Voldemort. For example, she could discover a way to rewrite the rules of magic that govern Voldemort's power.