One interesting plot twist could be that Harry actually has a hidden motive for joining Voldemort. Maybe he wants to learn the Dark Lord's secrets from the inside in order to ultimately defeat him. Another could be that his joining causes a rift among the Death Eaters, as some fear Harry's power and influence. Also, it might lead to a new form of magic being discovered as Harry combines his natural talent with the Dark Arts he learns from Voldemort.
One interesting plot twist could be that Harry Potter has to sacrifice his own magic to defeat Voldemort completely. In this scenario, after the final battle, Harry becomes an ordinary wizard - no more special powers. But he is okay with it as he has saved the wizarding world.
One idea could be that Harry, influenced by dark magic, forms an uneasy alliance with Voldemort. They could team up to take over the wizarding world in a different way than Voldemort originally planned, perhaps by ruling jointly. Another plot could involve Harry exploring the darker aspects of his own magic and using some of Voldemort's knowledge to do so, which causes a rift between him and his friends.
In certain fanfictions, a plot twist is that Harry doesn't kill Voldemort directly. Instead, he uses his love and friendship magic in a very unique way. He manages to make Voldemort see the error of his ways, and Voldemort sacrifices himself to undo all the evil he has done. This is a very different take from the usual 'kill or be killed' scenario. Another interesting plot twist could be that the killing of Voldemort reveals a long - hidden prophecy about Harry that has nothing to do with the original prophecy, and this new prophecy sets Harry on a whole new path of discovery and adventure.
One plot twist could be that Harry is actually a double agent. He pretends to be fully under Voldemort's control while secretly working against him. Another could be that Voldemort's training awakens a hidden power in Harry that is neither good nor bad, but something completely unexpected that changes the whole dynamic of the wizarding world.
A plot twist might be that when Harry goes to Voldemort, he discovers that Voldemort has a secret weakness. Maybe it's related to his family or some long - forgotten magic. Harry then has to decide whether to use this weakness against Voldemort or try to redeem him. This would add a whole new layer of complexity to the story. Another aspect could be that there are other forces at work, like a third - party wizard who is manipulating both Harry and Voldemort for their own gain, and Harry has to figure out who this is while dealing with Voldemort.
One interesting plot twist could be that Voldemort is secretly being controlled by an even more powerful dark force. It changes the whole dynamic as Harry now has to figure out not just how to defeat Voldemort, but also this other entity.
One possible plot twist could be that while captured, Harry discovers a hidden power within himself. Maybe it's a new type of magic that he unlocks due to the extreme situation. This power allows him to not only break free from his captors but also gives him an edge over Voldemort. Another aspect could be that the capture was part of a plan all along, by Dumbledore or some other wise wizard, to deceive Voldemort and get Harry in a position where he can deliver the final blow.
Maybe she discovers a hidden prophecy that states she has the power to either destroy Voldemort or become his most powerful ally. This would create a great internal struggle for her as she was raised by him but also has a sense of good. She might start to manipulate Voldemort into making decisions that would eventually lead to his downfall.
One possible plot twist could be that Harry struggles with his new identity. He might initially reject the idea but then gradually discover certain dark powers within him that he has to learn to control. This could lead to a conflict with his friends who are unsure whether to trust him.
One idea could be that Harry discovers a new spell that can penetrate Voldemort's dark magic defenses and uses it to shoot him. Another could be that Harry teams up with some unexpected allies, like the house - elves, who help him get in a position to shoot Voldemort. Maybe Harry has to sacrifice something important, like his invisibility cloak, to gain the power needed to fire the shot that will defeat Voldemort.