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.
It might be the Ministry of Magic itself. They are so afraid of a repeat of Voldemort's tyranny that they see Harry as a danger. They would try to arrest him or even execute him without really understanding that Harry has no intention of being like the original Voldemort. This would create a situation where Harry has to fight against the very institution that is supposed to protect the wizarding world, all while trying to prove that he is still on the side of good despite his new powers and appearance.
It could be Draco Malfoy. He might be jealous of Harry getting the teaching position and try to undermine him at every turn. Malfoy could spread rumors about Harry not being qualified or try to disrupt his classes in some devious ways.
It could be a rogue Death Eater who wants to take over Voldemort's position and sees Harry and Bellatrix's new - found alliance as a threat. For example, a Death Eater who was always jealous of Bellatrix's closeness to Voldemort.
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 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.
Maybe Harry has all of Voldemort's powers but uses them for good. He could start to look like Voldemort but with a kind heart, and he uses his new - found dark magic to fight off new threats that are even more powerful than Voldemort ever was.
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.
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 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.
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.
Hermione could be a main character. She might be trying to find a way to rescue Harry from the timeout situation. Ron could also be there, supporting Hermione in her efforts.