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.
In this fanfiction, Hermione could initially be in denial about Sirius being a Death Eater. She might think it's a ploy by Voldemort to mislead the Order of the Phoenix. So, she starts her own investigation, using her vast knowledge of magic and her sharp intellect to get to the bottom of it.
A great plot point could be that Sirius, as a Death Eater, is given a mission to target Hermione. But when he faces her, he can't bring himself to harm her because of their previous relationship. This internal struggle within Sirius and Hermione's attempts to bring him back to the light could form the core of the fanfiction.
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.
In a Hermione and Sirius family fanfiction, Hermione might end up marrying into the Black family. Since Sirius was an important part of the Black family before his estrangement, Hermione could find herself involved in the family's complex politics and traditions. She could use her knowledge of magic and her Muggle - born perspective to challenge some of the old - fashioned ideas within the family. Sirius, on the other hand, could be like a mentor to Hermione, teaching her about the more secretive and dangerous aspects of the wizarding world that she might not have known otherwise.
One possibility could be that Sirius sees Hermione's intelligence and determination and decides to take her under his wing. Maybe he realizes her potential and wants to protect her from the dangers in the wizarding world that he has faced himself.
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.
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.
Yes, it can be quite a popular concept in some fanfiction circles. The idea of turning a heroic character like Harry into a Death Eater is a very interesting and controversial twist. It allows for exploring different aspects of the Harry Potter universe, such as what would happen if the power of the Dark Side had swayed him. It also makes for some very intense and dramatic storylines as his relationships with other characters would be completely upended.
It's a rather concerning concept. In the 'Harry Potter' world, Hermione is a role model for many with her courage, knowledge, and tenacity. The idea of her being submissive to a Death Eater is not only out of character but also has the potential to introduce negative and disturbing ideas into the fanfiction community. Fanfiction should usually build on the positive aspects of the original work, not create such counter - intuitive and potentially offensive scenarios.