Another plot twist could be that the reason the Death Eaters save Harry is because Voldemort has been possessed by an even more evil entity. The Death Eaters realize that Harry is the only one who can defeat this new threat, so they go against their master to save the boy who lived.
Maybe the Death Eaters save Harry, but then they try to use him as a bargaining chip. They think they can trade Harry's safety for something they want, like amnesty or a powerful magical artifact. However, things don't go as planned and they end up having to truly protect him.
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.
Maybe he would be trained to be a super - powerful dark wizard. However, his natural good - heartedness, which he got from his parents, starts to shine through. This could lead to a conflict within the Death Eater community. He might be seen as a threat to their ideology and have to fight for his life against those who raised him.
Maybe Hermione discovers that one of the Death Eaters training her has a change of heart. This Death Eater then starts to help Hermione escape or undermine the other Death Eaters from within the group. It adds an interesting layer of complexity to the story.
A possible plotline could be that a Death Eater discovers Hermione has knowledge of a powerful spell or magic item that could help them overthrow Voldemort. So, they save her to get that information. For example, she might know of a way to break a powerful curse that Voldemort has placed on them.
Well, in a fanfic like this, it could be that the three Death Eaters have some ulterior motive for training Harry. They might think they can turn him to their side and use his powers for their evil plans. Harry, on the other hand, could be biding his time, learning their secrets so he can later use it against them. For example, he could be learning about their hideouts or how they communicate, all while pretending to go along with their training.
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.
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.
In a 'Harry kills all Death Eaters' fanfiction, the plot could start with Harry getting fed up with the constant threat the Death Eaters pose. He could go on a solo mission to hunt them down one by one. Along the way, he might face some moral dilemmas as he realizes that some Death Eaters might have families or could potentially be redeemed. But in the end, he decides that the only way to ensure peace is to eliminate them all. This could lead to a series of intense battles in different wizarding locations, and Harry could gain new allies or use some unexpected magical artifacts to achieve his goal.
One possible plot twist could be that the daughter of death is initially thought to be on the side of evil but is actually a key to saving the wizarding world. Another could be that she has a secret connection to a major character from the original series that changes everything. Maybe she's related to Dumbledore in some way.
A plot twist could be that Ginny is not really Ginny. The Death Eaters might have kidnapped a disguised decoy while the real Ginny is off on a secret mission. Or, perhaps the place where she is being held has some ancient magic that starts to affect the Death Eaters in unexpected ways, like making them see their worst fears. This could give Ginny an opportunity to escape. Also, it could be that Ginny's kidnapping was a set - up by someone within the Order of the Phoenix who has been secretly working for the Death Eaters all along, but Ginny uncovers this plot and uses it to her advantage.