In some fanfiction, Sirius Black might be wrongly portrayed as a Death Eater for the sake of creating an alternative or dark storyline. But in the original 'Harry Potter' series, he is not a Death Eater at all. He was framed by Peter Pettigrew.
Some fanfiction writers might make Sirius Black a Death Eater to explore the idea of redemption in a different way. If he starts as a Death Eater, then his journey to the side of good could be a more complex and interesting story. Also, it could be a way to add more mystery to the 'Harry Potter' universe by changing the nature of a familiar character.
One interesting Sirius Black as a Death Eater fanfiction could be 'The Darkened Path of Sirius'. In this story, Sirius might be forced into the Death Eater ranks due to some dark family secrets. Another could be 'Sirius's Fall and Redemption'. It might explore how being a Death Eater changes him and his journey back to the good side. And 'The Alternative Sirius' could show a world where Sirius is a Death Eater from the start, changing the whole dynamic of the Harry Potter universe.
One possible scenario in such a fanfiction could be that Sirius is somehow forced or tricked into becoming a Death Eater. Hermione might then try to rescue him from the dark side. Maybe she discovers some ancient magic that can reverse the dark mark's influence on him.
Another idea is that Sirius, as a Death Eater, has some information that Hermione needs to defeat Voldemort. She has to convince him to give it to her while also dealing with his newfound allegiance. It could be a race against time as other Death Eaters suspect Sirius of being unfaithful to their cause, and Hermione has to protect him while also getting the crucial information.
Fanfictions about Sirius Black as a Death Eater can change the original narrative significantly. For example, it can change the perception of the Marauders. Instead of being the group of friends we know, they might be more divided if Sirius is a Death Eater. Also, Harry's view of his father's friend would be completely different. He might not trust Sirius at all.
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.
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.
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.
In some fanfictions, Percy Weasley may be depicted as a Death Eater, but this is a departure from the original canon. Fanfiction allows for creative reinterpretations, and some authors might explore this dark turn for his character to add drama and new plotlines.