A possible plot twist could be that Hermione gets turned into a Death Eater against her will, and Draco has to help her break free from the dark magic controlling her. This would completely reverse their typical roles.
One interesting plot twist could be that the Death Eaters have a prophecy about Hermione that Draco overhears. The prophecy states that she will either destroy the Death Eaters or lead them to ultimate victory. This makes Draco determined to either convert her to their side or kill her. But as he gets closer to her, he realizes that the prophecy might be misinterpreted. This leads to a whole new set of adventures and a change in their relationship as they try to figure out the true meaning of the prophecy together.
Maybe Draco discovers that Hermione is actually related to a powerful dark wizard in some convoluted way. This revelation shakes his view of her and the whole wizarding world order. It also adds a new layer of complexity to their relationship as he tries to figure out how to deal with this new information while still being a Death Eater. For Hermione, it's a shock to her identity and she has to come to terms with it while also dealing with Draco's changing attitude towards her.
One idea could be that Draco and Hermione are forced to work together undercover among the Death Eaters. They start off distrusting each other but as they face various challenges within the Death Eater ranks, they begin to understand each other better. For example, they might have to find a way to stop a particularly heinous Death Eater plan while pretending to be on their side.
A great plot twist could be that the Death Eater she's betrothed to is actually a double agent himself. All along, Hermione thought she was the one with the upper hand in the undercover operation, but it turns out he's been playing a long - game too. This would completely change the power dynamic between them and add a whole new layer of complexity to their relationship.
One possible plot twist could be that the Death Eater she marries is secretly a double agent. He might be working against Voldemort from within the Death Eater ranks and Hermione discovers this. Their relationship could start as a way to get closer to the Death Eater secrets but then turn into real love.
Perhaps Hermione's transformation into a Death Eater is a result of a time - travel accident. She goes back in time, something goes wrong, and she ends up being influenced by the Death Eaters of that era. This would create a complex web of cause and effect in the fanfiction.
One potential plot twist could be that Hermione's father was actually a double agent. So, while everyone thinks she's the daughter of a Death Eater, in reality, he was working for the good side all along. Another twist might be that Hermione discovers she has some special powers because of her heritage that can be used to defeat Voldemort in a unique way. For example, she could have an innate understanding of dark magic that helps her counter the Death Eaters' spells.
Another aspect could be an unexpected bond forming between Hermione and the Death Eater. As they are forced to spend time together, they might start to understand each other on a deeper level. This could lead to a moral dilemma for the Death Eater, especially if he starts to develop feelings for Hermione. He would then have to choose between his loyalty to the Death Eaters and his new - found connection with her.
Well, it could be that Draco is forced to follow the Death Eater path while Hermione is fighting against them. But then, they might get thrown into some situations together, like being trapped in a place. Draco realizes that Hermione is not just a Mudblood in the sense that he was taught, and they start to form an unlikely bond. Their relationship could be full of tension, with Hermione constantly trying to convert Draco or at least make him see the wrong in what he's doing, and Draco struggling between his loyalty to his family's Death Eater ideals and his growing feelings for Hermione.
One interesting plot twist could be that Draco saves Hermione not out of pure altruism but because he discovers a dark plot against the wizarding world that Hermione unknowingly holds the key to solving. So, by saving her, he's also saving himself and his family from a greater threat.
One plot idea could be a werewolf Death Eater who is forced to work with Hermione under some strange circumstances. Maybe they are both on a mission to retrieve a powerful magical artifact, and along the way, the werewolf starts to question his loyalty to the Death Eaters as Hermione shows him the value of kindness and justice.
One idea could be Hermione discovers she has a long - lost pureblood heritage that has some ties to the Death Eaters. Maybe her family was involved in some secret Death Eater plot in the past, and she has to uncover the truth to clear her family name.