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.
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.
There could be a fanfiction that delves into the magical afterlife concept in relation to Sirius Black's death. It could be about his spirit lingering in a limbo state, trying to communicate with the living. This would add a new dimension to the story, as it might involve some new magical elements and how the wizarding world deals with spirits and the afterlife.
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.
In many fanfictions, Harry becomes more withdrawn after Sirius' death. He might lose his trust in the adults around him more easily, feeling that they couldn't protect Sirius, so he tries to be more self - reliant. He also could become more brooding and angry, his emotions more volatile as he grapples with the sudden and tragic loss.
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.
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.
One common change is that he becomes moodier. The loss of Sirius is a huge blow, and he may swing between being extremely sad and angry. Another change could be that he is less likely to take risks without thinking. Sirius' death makes him more cautious as he realizes the consequences of recklessness.
One really heartbreaking Sirius death fanfiction might be those that focus on Harry's reaction in great detail. It could show Harry's disbelief and his overwhelming sense of loss, as Sirius was like a father figure to him. The way the author describes Harry's internal struggle, like his anger at himself for not being able to save Sirius and his grief that seems to swallow him whole, makes it truly sad.