One interesting plotline could be the backstories of the Death Eaters. Maybe exploring how they were recruited by Voldemort, like Bellatrix's extreme loyalty and what led her to that. Another could be their interactions outside of the main battles in the original series. For example, how they deal with the power struggles among themselves when Voldemort is not around.
In some fanfictions, a plotline could be the redemption of a Death Eater. Imagine a Death Eater who starts to question Voldemort's ways after seeing the destruction caused. They might try to secretly help the Order of the Phoenix. This would add a lot of drama and moral complexity to the story. Also, the relationships between different Death Eaters families could be explored, like the Malfoys and their influence on other Death Eater families.
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.
One interesting plot could be a group of young wizards and witches forming an underground resistance against the Death Eaters. They might use unique spells and magical artifacts that are not well - known in the original series to outwit the Death Eaters.
One plotline could be a Death Eater who was forced into Voldemort's service but secretly despises him. This Death Eater could start helping Harry by providing him with inside information about Voldemort's defenses. For example, they could tell Harry about the secret passages in the Death Eaters' lair.
Well, in such fanfiction, a common plotline is Harry being trained in secret Death Eater techniques. This would make him a formidable wizard in a different way than in the original books. He could also be used as a pawn by the Death Eaters against Dumbledore and the Order of the Phoenix, leading to some intense battles and strategic moves. And his connection to Voldemort might be even more muddled, as the Death Eaters may try to manipulate that for their own gain.
One possible plotline could be Harry using new and powerful spells he's learned in secret to take on the Death Eaters one by one. Maybe he discovers a spell that can disarm and immobilize them all at once, leading to their quick defeat.
It's also possible that some of the Death Eaters themselves could be main characters in a sense. For example, Bellatrix Lestrange could be a central figure if the fanfiction focuses on the power struggle within the Death Eater ranks during Harry's hunt, or if there is some backstory or different take on her relationship with Voldemort and the other Death Eaters.
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.
Well, a possible plotline is that they discover a Horcrux in a place that was previously thought to be just a mythical or lost location. Maybe it's hidden in an ancient wizarding ruin that has been long forgotten. As they search for this Horcrux, they encounter all kinds of magical creatures and traps. And also, their relationship might develop romantically during the hunt. They share moments of fear and triumph which bring them closer together.
One possible plotline could be a story of redemption. Maybe a Death Eater starts to question their beliefs after getting to know Hermione on a deeper level. For example, they could be forced to work together on a mission and gradually see the good in her, leading to a change in their ways.
One possible plotline could be Harry struggling to come to terms with his unique parentage. He might face discrimination or fear from others in the wizarding world who don't understand. Another could be his relationship with Death. Maybe Death has certain tasks or lessons for Harry that he must fulfill or learn. And also, his friends' reactions to this revelation would be interesting. Hermione might be eager to research about it while Ron could be a bit more skeptical at first.