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.
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.
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.
Well, I'm not sure of specific 'Harry Potteradipted by Voldemort' fanfics off the top of my head. But you can try looking on some popular fanfic websites like FanFiction.net or Archive of Our Own. They usually have a wide range of fanfics, and you might be able to find what you're looking for by using relevant search terms.
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 the goblins turn against Harry midway through his visit. They could be influenced by a dark wizard who has a grudge against Harry. This would completely change the course of his visit and force him to find a new way to achieve his goals in Gringotts. Another possibility is that the vault contains a magical item that has a mind of its own and tries to possess Harry when he touches it.
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, a plot twist might be that Draco, who is supposed to be Voldemort's pet, actually starts to have a strange influence on Voldemort. Maybe Draco's ideas or his unique perspective begin to make Voldemort question some of his own actions or beliefs. Or perhaps there is a revelation that Draco was put in this 'pet' position by someone else as part of a bigger plan, and Voldemort was unknowingly a part of it.
Perhaps Voldemort discovers that there is a greater threat out there. In the fanfic, it could be that another dark force is emerging, and Voldemort sees Harry as an ally of sorts against this new threat. So, he decides to protect Harry in order to deal with this common enemy.
One possible plot twist could be that the girl who is pregnant has some sort of magical ability related to the pregnancy that no one expected. Maybe she can communicate with the unborn child in a magical way. Another could be that Harry discovers he's not the real father, but due to some magical mix - up, he has to take on the role anyway.