One possible plot could be Hermione's internal struggle. She is a strong - willed character, so being betrothed to a Death Eater would go against all her beliefs. She might try to find a way to break the betrothal while also having to deal with the Death Eater's family and their dark magic - influenced ways. For example, she could be constantly trying to outwit them in a battle of wits, using her intelligence to avoid any dark magic they might try to force on her.
Bellatrix Lestrange's younger brother or some other minor Death Eater character could be used. This would allow for the creation of a new character with his own motives and backstory. He could be betrothed to Hermione as part of a larger plot by Voldemort to gain control over Hermione's knowledge and magic. This would put Hermione in a very difficult position as she would be tied to a very dark and dangerous family.
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.
Well, it could be a time - travel fanfiction. The Death Eater might be from a different era, one where he hadn't fully committed to the dark side yet. Hermione gets betrothed to him in the past, and through her knowledge of the future, she tries to prevent him from becoming a full - fledged Death Eater. This would create a very interesting dynamic as she battles against the forces that are pulling him towards the dark side while also dealing with the social implications of being betrothed to someone with such a dark future.
Often, there are plots where Hermione is a Death Eater as part of a deal to save her friends. Maybe she makes a pact with a powerful Death Eater to spare Harry and Ron in exchange for her loyalty. There are also stories where she becomes a Death Eater due to a family secret or connection that ties her to the dark side. This then leads to a journey of self - discovery as she tries to reconcile her new identity with her old values.
One common plot is that she might be a double agent. Pretending to be a Death Eater to get inside information for the good side. For example, she could be using her intelligence to outwit Voldemort and his followers from within. Another plot could be that she was brainwashed as a child by a hidden Death Eater and only discovers her true identity later. It's a dark and twisted take on her character.
One interesting plot could be Hermione being forced to work with a Death Eater undercover. This creates a lot of tension as she has to hide her true feelings and intentions. Another might be a story where a Death Eater has a change of heart because of Hermione's influence. It shows the power of her character. And then there could be a plot where Hermione discovers a hidden group of Death Eaters within Hogwarts and has to figure out how to expose them without getting caught herself.
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 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.
One interesting plotline could be Hermione using her intelligence to try and reform some of the Death Eaters. She might engage in deep philosophical debates with them, trying to show them the error of their ways. For instance, she could target younger Death Eaters who might be more susceptible to change.
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.