There could be a plot twist where Hermione is the key to Voldemort's redemption. Since she's his niece, she might have some special connection to him that could potentially turn him from the dark side. Or perhaps it turns out that Voldemort isn't as evil as he seems in relation to Hermione, and there are other forces manipulating him that Hermione has to uncover.
A really unexpected plot twist could be that Hermione's family was actually trying to use her to stop Voldemort from within. They knew of their relation all along and sent Hermione to Hogwarts to befriend Harry Potter with the intention of ultimately defeating Voldemort. Or it could be that Voldemort has a prophecy related to Hermione that he believes will either make him invincible or destroy him, which adds a whole new level of complexity to the story.
One possible plot twist could be that Riddle has a secret connection to Hermione's family. This would completely change her perspective on him and how she approaches the situation. Another could be that Draco is actually working undercover for a good cause all along and has been trying to get close to Riddle to expose him. And perhaps Hermione discovers a new form of magic that can only be harnessed with the help of both Draco and Riddle, forcing them into an uneasy cooperation.
One common plot twist is that Hermione discovers her heritage at a crucial moment, like when she's about to be attacked by Death Eaters and suddenly some hidden power related to her being Voldemort's heir emerges. Another is that she has to struggle with the moral implications of being related to such an evil figure.
One interesting plot twist could be Hermione struggling with her dark heritage while trying to uphold her Gryffindor values. Maybe she discovers a hidden power within her that she initially fears is evil but later uses for good.
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.
A really interesting plot twist might be that Voldemort and Percy are forced to team up against a common enemy. They realize that there is a greater threat that neither of them can face alone. This would be a huge change in their relationship as they are initially enemies. Or perhaps Percy discovers that he has some latent wizarding abilities due to his godly heritage, which completely changes the dynamics of the fight as he can now use both his demigod powers and magic.
A plot twist could be that Android 18 has some hidden Saiyan DNA. This would completely change the dynamic between her and Vegeta. They might then explore their newfound connection related to their Saiyan heritage together. Maybe she could even unlock some Saiyan - like powers, which would be a huge shock to everyone in the Dragon Ball Z universe.
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.
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.
In one possible plot, Voldemort might kidnap Hermione to use her knowledge of magic against Harry. He could try to force her to reveal secrets about Dumbledore's plans or the location of certain powerful magical artifacts. Hermione, being strong - willed, would resist and try to outwit Voldemort using her intelligence.
A major plot twist could be that one of the beloved original characters is actually a double agent for a new, hidden dark force. This would shatter the trust within the wizarding community. Another twist could be that the magic itself starts to malfunction in strange ways. Spells that have always been reliable no longer work as expected, and it's up to the new generation, with the guidance of the old heroes, to figure out what's wrong. Maybe the Sorting Hat has started to give different sorting results based on some new, unknown criteria.