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.
Lucius Malfoy could also be a candidate in a more alternate - universe type of fanfiction. He is a well - known Death Eater. A betrothal to Hermione could be part of some political maneuvering within the wizarding world. Maybe the Malfoy family wants to use Hermione's intelligence and influence to their advantage in the post - Voldemort era, or perhaps it was a deal made during Voldemort's reign to secure the loyalty of Hermione's family (if they were to be coerced into it).
Bellatrix Lestrange is also a likely candidate. Her encounter with Hermione would be extremely intense. Given her loyalty to Voldemort and her rather unhinged nature, she would likely pose a serious threat to Hermione, but Hermione might also find a way to use Bellatrix's madness against her.
There's also a possibility that it could be a minor Death Eater that we don't know much about from the original series. The author of the fanfiction could create a new character who was a Death Eater but is now having doubts. Hermione could be the one to find this person and start their journey of change. This would give the author a lot of creative freedom to develop the character's backstory and their relationship with Hermione.
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.
It could be someone who has a hidden good side, like Draco Malfoy. He was from a Death Eater family but showed signs of not fully embracing the dark side. Maybe in this fanfiction, he decides to protect Hermione for some unforeseen reason, perhaps because he has developed a new understanding of the value of life and friendship.
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.
One possible motive could be a deep - seated dissatisfaction with the wizarding world's status quo. Maybe she felt that the Ministry of Magic was corrupt and that the Death Eaters had a better vision for change, although a very dark one. Another could be some sort of personal vendetta against someone in the Order of the Phoenix or Hogwarts faculty that made her side with the Death Eaters.
It could also be that Hermione is on a mission to gather information. By helping the Death Eater, she hopes to gain insider knowledge about the Death Eater's group or their plans. This would be a strategic move on her part, using the situation to her advantage in the fight against the dark forces.
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.
In this type of fanfiction, the relationship between Hermione and the Death Eater could be very complex. Maybe it starts as a hostile one, but as the story progresses, they might find some common ground or mutual understanding. It could also be a story of redemption for the Death Eater through Hermione's influence.
It's a really strange concept. In the original 'Harry Potter' universe, Hermione is firmly on the side of good. But in fanfiction, if she's a Death Eater, it could be a result of a time - travel gone wrong. She might have made a different choice in an alternate timeline that led her down the dark path. But again, this is very much against the essence of her character.