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.
Hermione's character could be developed by showing her internal struggle. If she has a pureblood connection to Death Eaters, she might question her own identity. For example, she could struggle with the dark side of her family history while still holding onto her values of justice and knowledge.
In 'bella is a pureblood and death eater' fanfiction, the theme of identity is prevalent. As a pureblood in the Death Eater circle, she has to constantly reaffirm her identity. Is she just a tool of the Dark Lord? Or does she have her own motives? Also, the theme of secrecy and betrayal often comes up. She may have to keep secrets from her family or face betrayal from her fellow Death Eaters due to her pureblood status.
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.
Well, in fanfiction, the writer might be trying to create a more complex and subversive narrative. Hermione as a pureblood Death Eater could be part of a larger plot to show how even the most unlikely characters can be corrupted. It could also be a way to explore the grey areas between good and evil in the wizarding world. Maybe there are new political or magical forces at play that force Hermione to side with the Death Eaters, but this is a huge departure from her canon self.
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.
One popular 'bella is a pureblood and death eater' fanfiction is 'The Dark Legacy of Bella'. It delves into her life as a pureblood in the Death Eater ranks, exploring her relationships with other dark wizards. Another one could be 'Bella's Ascent in the Dark World' which focuses on her rise within the Death Eater community and how her pureblood status aids her.
The relationship between Hermione and her friends would be shattered. Harry and Ron would be in shock and disbelief. This would also lead to a different path for Hogwarts. With Hermione on the side of the Death Eaters, she could potentially influence the school in a negative way, changing the educational system and the safety of the students. The overall tone of the wizarding world would become much darker as Hermione's character was often associated with hope and light in the original story.