Well, one possibility is that Harry would be forced to learn dark magic. The Death Eaters might try to mold him into one of their own, but Harry's strong sense of good might resist. He could secretly plot against them while pretending to go along with their ways.
A possible motive could be self - preservation. If they believe that Voldemort's downfall is inevitable and that by saving Harry, they can gain some form of leniency from the side that opposes Voldemort. They might think that Harry has enough influence to spare their lives if they help him.
His magical education would be very different. Instead of learning at Hogwarts, he might be taught in some dark and secret location by Death Eater instructors. They would likely focus on dark spells and hexes.
Voldemort himself, of course, is the most prominent in a sense. But in the fanfiction, his relationship with the other Death Eaters could be different. He might be more distrustful or have different expectations of them. And then there's Barty Crouch Jr. He could be a major player in the fanfiction, with his skills in disguise and dark magic. His actions could drive some of the major plotlines in the fanfiction.
Perhaps there is a prophecy that the Death Eaters misinterpret at first. They think killing Harry is the right thing, but later they discover that saving him is actually crucial for their own survival or for some greater good in the wizarding world. So they decide to go against Voldemort's orders and save Harry. For instance, they find out that Harry has some power that can prevent a much greater evil from emerging, which would also threaten the Death Eaters.
A fanfiction involving Hermione and Death Eaters could focus on a moral dilemma. For example, if Hermione has the chance to save a Death Eater's life, would she do it? This could explore her character's sense of right and wrong. It could also be about how she endures the psychological torture that the Death Eaters might inflict on her, showing her strength and resilience.
Well, perhaps in this fanfiction, there's a big misunderstanding. Harry might think that by infiltrating the Death Eaters as one of them, he can actually bring them down from the inside. So he pretends to join them, but things get out of hand as he has to constantly prove his loyalty to the dark side, which is really difficult for him.
They might be outraged. Fanfiction often plays with the established lore and characters, and the Death Eaters, being firm believers in pure - blood supremacy and the Dark Lord's ways, would likely be horrified by any deviation from what they see as the 'true' story. They could see it as a mockery of their beliefs and actions.
Well, one possibility could be that he is forced to join them under some kind of dark magic threat. Maybe Voldemort has found a way to control his mind and he has no choice but to follow their orders. This would lead to a lot of internal conflict for Harry as he struggles with his own values.
One common theme is heroism. Harry and other characters show great courage as they take on the dangerous task of hunting Death Eaters. They are willing to risk their lives to make the wizarding world a safer place.