A powerful dark wizard from a far - off land could also be the main antagonist. He might come to Hogwarts with the intention of stealing the secrets of the Philosopher's Stone or some other powerful magical artifact. He has his own brand of dark magic that is completely different from Voldemort's, which makes him a very difficult opponent to face in the 'reborn as Harry Potter fanfiction'.
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.
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.
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 follower of the God of Death who misinterprets the God's will and tries to bring chaos to the wizarding world. They might see Harry as an obstacle to their plans and try to eliminate him.
Perhaps a dark wizard who is jealous of Harry's new power. This wizard could be someone who has long been in the shadows, waiting for the right moment to strike. He might see Harry as a threat to his own plans of taking over the wizarding world.
Perhaps it's a corrupted Hogwarts professor. In this fanfiction, a teacher could have been influenced by the dark side and decides to oppose Harry's new path. They could be using their position at Hogwarts to spread dark magic teachings or to manipulate students against Harry. They would be a more insidious antagonist as they are operating from within the very institution that is supposed to be a safe haven for Harry.
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.
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.