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.
These stories usually show Hermione as being very resourceful. When she gets to know Death Eaters, she is constantly looking for ways to use the situation to her benefit or the benefit of the wizarding world. She might study their magic, their behavior, and their weaknesses. Her character is also depicted as having a strong sense of justice. So, even if she starts to understand a Death Eater's motives, she doesn't excuse their actions if they are wrong. She may try to rehabilitate them, but she won't tolerate evil deeds.
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.
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.
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.
Another idea could be that she is somehow brainwashed. There could be a dark spell or a powerful magical object that alters her perception and makes her believe in the Death Eater ideology. In this scenario, her friends would have to find a way to break the spell and bring her back to her true self.
One possible aspect of this fanfiction could be Hermione's encounter with an unexpected Death Eater. Maybe she stumbles upon one during a mission or while exploring a hidden area. This encounter could lead to a series of thrilling events, like a battle of wits or a dangerous duel.
In some fanfictions, Hermione as a Death Eater is a very interesting and often controversial portrayal. Sometimes, it might show her being forced into the role due to some dark magic or blackmail. This completely flips her good - girl image from the original series and allows for a very different exploration of her character.
One possibility in this fanfiction could be that Hermione is forced to become a Death Eater due to a powerful dark magic spell that controls her mind. This would lead to a great internal struggle as her original values clash with the cruel and dark ways of the Death Eaters.