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 possible plot twist could be that the training is all an illusion created by a third party. This third party has its own agenda, whether it's to overthrow both Harry and Voldemort or to use their power for something else. Also, Harry could learn a new form of magic from Voldemort that allows him to time - travel or access other dimensions, completely changing the course of the story.
Maybe Harry discovers that Voldemort has a secret motive for training him. For example, Voldemort needs Harry to complete a certain spell or task that he himself cannot do. Or perhaps, during the training, Harry finds out that there is a deeper connection between him and Voldemort that has been hidden all along, like they are related in a way no one expected.
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.
A third plot twist could involve a character we least expect coming to Harry's rescue. Perhaps a Slytherin student who has been secretly against Voldemort all along. This student could have some inside knowledge about Voldemort's lair and manages to free Harry, leading to a change in the dynamic between the houses at Hogwarts.
A really interesting plot twist might be that Voldemort is being controlled by an even darker force, and Harry has to figure out a way to break that control while being held captive. Or perhaps Harry's friends, thinking he is dead, accidentally start a chain of events that end up helping Harry escape in a very unexpected way. For example, Hermione might cast a spell that was supposed to be for protection but ends up creating a portal right in Voldemort's lair.
Well, a really interesting plot twist could be that when Voldemort controls Harry, Harry's personality starts to merge with Voldemort's in unexpected ways. For example, Harry might start to understand Voldemort's motives and see some truth in his views about the wizarding world, which creates an internal conflict for Harry. Another plot twist could be that the magic used to control Harry has a side - effect that it also starts to affect Voldemort's own magic in unforeseen ways. Maybe it makes Voldemort more vulnerable or changes his appearance. And there's always the chance that the control is not just a simple spell but a complex magical ritual that was set in motion long ago, and Harry has to figure out how to reverse the entire ritual to break free.
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.
Perhaps a major plot twist could be that Voldemort actually has a change of heart after 'adapting' Harry. He sees something in Harry that makes him question his own evil ways. This could lead to a strange alliance between the two where they try to find a new way to rule the wizarding world that is less about destruction and more about a new form of magic governance. It would be a very unexpected turn considering Voldemort's character in the original series.
One possible plot point could be the discovery of this relationship. Maybe Harry finds some hidden magical artifact or document that reveals the truth. Another could be the reaction of the wizarding world. Would they still see Harry as a hero or would they be suspicious of him? Also, Voldemort's reaction if he were to find out before his death could be interesting. He might try to use Harry for his own ends in a different way than in the original story.
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.
One possible plot twist could be that the 'forced slave' aspect is actually a ruse by Dumbledore to get Harry close enough to Voldemort to kill him. Harry discovers this later and has to deal with his feelings of betrayal. Another could be that when Harry kills Voldemort, he absorbs some of Voldemort's power and has to struggle with the dark side that comes with it. A third plot twist might be that the force controlling Harry as a slave is actually an ancient magic that has its own motives for wanting Voldemort dead, and Harry has to break free from this magic's control after killing Voldemort.
Perhaps in this fanfiction, Harry discovers some dark secret about the Order of the Phoenix or Dumbledore that makes him lose faith in the side he has always been on. This, combined with Voldemort's manipulation, could lead him to side with the Dark Lord. For example, if he finds out that Dumbledore has been using him all along without his full knowledge, he might be so disillusioned that he turns to Voldemort as an alternative.