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.
Well, in fanfiction, anything can happen. Hermione as a Death Eater could potentially be part of a complex plot where the author wants to explore the idea of good turning bad. Maybe she was disappointed with the Order of Phoenix for some reason and was swayed by the Death Eaters' false promises. However, it's hard to imagine Hermione abandoning her values so easily. In most cases, this is just a creative and often shocking idea used by fanfiction writers to draw readers in and make them think about the characters in a new light.
It's a really wild concept in fanfic. Hermione is always associated with knowledge, justice, and the side of good. However, some fanfic authors might be trying to subvert the normal expectations. But in the canon, there's no way she could be a Death Eater. She detests everything they stand for, from their pure - blood mania to their use of dark magic.
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.
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.
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.
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.
In fanfic, a Hermione Granger and Death Eater pairing can be quite an interesting and controversial concept. It might explore the idea of redemption for the Death Eater through Hermione's influence. For example, if it was a pairing with Draco Malfoy (who had Death Eater ties), fanfic could show how Hermione's strong will and intelligence could change his views over time. However, it also challenges the traditional good - versus - evil dynamic in the Harry Potter universe.
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 can completely shift the narrative. It adds an element of conflict right from the start. The fanfic would have to deal with the inherent differences between Hermione's values and those of a Death Eater. For instance, Hermione values equality among wizards and witches, while Death Eaters often believe in pure - blood supremacy.
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.