A possible plot twist is that the forced slave Harry is actually in an alternate dimension where the rules of magic are different. When he kills Voldemort, he gets stuck in this dimension and has to find a way to get back to his own world. Also, it could be that the forced slave status was a test by some powerful magical beings. Harry passing the test by killing Voldemort means he now has a new mission. Maybe he has to help rebuild the wizarding world in a new way. Another plot twist could be that Harry's forced slave state was caused by a family secret. After killing Voldemort, he discovers this secret and has to come to terms with his family's dark past while also trying to move forward with his life.
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.
Well, perhaps a plot twist could be that the forced slave condition is related to a prophecy. Harry is forced into this role because of a misinterpreted prophecy, and when he kills Voldemort, he realizes that the whole prophecy thing was a set - up by a group of dark wizards who wanted to use Harry as a weapon. Another idea is that the power that makes Harry a forced slave is transferred to Voldemort when he kills him, and then Voldemort comes back in a new form, and Harry has to find a way to defeat him again. Or, the forced slave situation could be a result of a spell cast on Harry by a character we least expect, like a seemingly innocent Hogwarts professor, and after killing Voldemort, Harry has to expose this professor's true nature.
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.
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.
One possible plot twist could be that Harry was set up. Maybe there was a powerful dark wizard who wanted to get rid of Harry and framed him for Voldemort's death in a way that made it seem like a cold - blooded murder rather than a heroic act. Another twist could be that the wizarding world has turned against Harry because they fear his power after defeating Voldemort and use this arrest as a way to control him.
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 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.
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.
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 twist could be that Harry discovers the marriage is part of a dark wizard's plan to gain control over him. Maybe the dark wizard wants to use Harry's fame and power within the wizarding world for their own evil deeds. Another twist could be that the person he is forced to marry is actually an undercover ally who is trying to protect Harry from a greater threat. For example, she might be pretending to be part of the forced marriage scheme but is actually working with Dumbledore's old network to keep Harry safe.
In such a fanfiction, perhaps the plot could involve fem Harry using her wits to outsmart Voldemort while in the guise of a slave. She might be able to gather information about his plans and pass it on to the Order of the Phoenix. But it's a very dark and unorthodox take on the story. Usually, the Harry Potter story is about good versus evil in a more straightforward way, and this would be a major deviation. There could also be a plotline where Harry tries to find a way to break the slave bond through some long - forgotten magic or ancient spell, which would add an element of mystery and adventure to this rather dark premise.
One possible plot development could be that Harry tries to find a way to escape the marriage. She might use her magical skills and the help of her friends to break free from Voldemort's control. Another could be that she pretends to go along with the marriage while secretly planning to undermine Voldemort from within.