Well, one possibility could be that Harry stumbles upon Fleur while she's being cornered by Death Eaters during a battle. He quickly uses a powerful disarming spell to take out the first Death Eater's wand. Then, he creates a diversion using a Patronus to confuse the rest and whisks Fleur away to safety.
Magic is essential in this scenario. Harry might have a special magical ability or connection that gives him an edge in saving Fleur. For instance, he could have a connection to the magic of the place where they are. If it's a forest, he could call upon the magic of the trees to help him. The trees could come alive and trap the Death Eaters, allowing Harry to rescue Fleur. This would show how in the wizarding world, magic is not just about spells from a wand but also about the connection to the surrounding magical environment.
One consequence could be a major shift in the wizarding world's perception of good and evil. If Harry saves the Death Eaters, people would have to re - evaluate what it means to be a 'bad' wizard. There would be a lot of distrust at first, but over time, it could lead to a more forgiving society.
One way this could happen is if Harry Potter discovers a new form of magic that can reverse the dark magic influencing the Death Eaters. Maybe he finds a powerful spell during his exploration of ancient magic that can purify their souls and free them from Voldemort's control.
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.
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.
Fleur could be shown as more independent and brave. Since she's taking it upon herself to save Harry, it shows her courage. Maybe she becomes more confident in her own abilities outside of the Triwizard Tournament context.
Well, perhaps Fleur uses her Veela charm to intimidate the Dursleys into letting Harry go. She could arrive at Privet Drive in a blaze of magical beauty and power, and the Dursleys, being the cowards they are, would be too scared to oppose her when she demands Harry come with her.
In some fanfictions, Fleur might use her Veela charm to intimidate the Dursleys into letting Harry go. The Dursleys, being easily cowed by the extraordinary, would be no match for Fleur's alluring yet powerful presence.
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.