Perhaps in this fanfiction, the betrothal is a ruse. Hermione is actually working undercover to bring down the remaining Death Eater cells. By getting betrothed to a Death Eater, she gains access to their inner circle. She has to pretend to fall in love with him while secretly passing information to the Order of the Phoenix or other good - wizards. This would be a really exciting and dangerous plot, full of twists and turns as she has to constantly be on guard not to blow her cover.
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.
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.
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.
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.
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.
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.
It changes the storyline by introducing an unexpected ally. Hermione, who was used to seeing Death Eaters as pure evil, now has to reevaluate her views. This can lead to new alliances and different ways of fighting against Voldemort. For example, she might use the Death Eater's inside knowledge of Voldemort's plans.
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 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.
In some fanfictions, Hermione might be a Death Eater spy due to a complex backstory. Maybe she was blackmailed. For example, if the Death Eaters threatened to harm her Muggle - born parents, she could be forced to spy for them. This would be a huge deviation from her canon character, but in the realm of fanfiction, it allows for exploring different character arcs and moral dilemmas.
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.