Their view could become more confused. Fanfiction can present all sorts of alternate universes and character developments. The Death Eaters, used to a very specific view of the wizarding world under Voldemort, would find it hard to accept different versions. They might see the wizarding world in fanfiction as a chaotic and wrong place, which would make them double down on their own views to feel more secure.
Their view might expand. They'd see different takes on the wizarding world that they hadn't considered before.
It could lead to a more peaceful co - existence. If Harry befriends Death Eaters, he might be able to convince them to give up their dark ways. This could result in a new era in the wizarding world where former Death Eaters and the rest of the wizarding community work together to rebuild and reform. For instance, they could help in rehabilitating other dark wizards.
The wizarding world would be in shock. Harry was seen as the 'Chosen One' to defeat Voldemort. If he joined the Death Eaters, the Order of the Phoenix would feel betrayed. They might try to rescue him or think he's been an undercover agent all along. Hogwarts would be in chaos, and students would be confused and scared.
His view could become more open - minded. He'd realize there are other ways to approach the supernatural world. For example, the way the hunters in Supernatural use weapons instead of wands might make him think about alternative forms of magic defense.
It might not change at all. They are so closed - off to the idea of magic that even fanfiction about it would seem like pure fantasy to them.
The wizarding world would be in chaos. Harry has been seen as the 'Chosen One' to defeat Voldemort. If he becomes a Death Eater, the hope that the wizarding world has placed in him would be shattered. People would lose faith in Dumbledore's judgment for believing in Harry all this time. The Order of the Phoenix would be severely demoralized as they have been fighting for Harry's cause.
After reading the fanfiction, Percy could have a more in - depth understanding of the wizarding world. He might start to think about how the social structure in the wizarding world compares to that in his demigod world. He might also be more curious about the history and lore of the wizarding world, and how it developed compared to his own world of Greek gods and monsters. He could even start to imagine what it would be like to be a wizard, with all the unique spells and magical items, and how that would change his approach to dealing with his own problems.
The Weasleys' understanding could be both broadened and a bit muddled. On one hand, they might see new interpretations of old magic or different ways characters could have interacted. For example, if the fanfiction explored a different side of Snape's actions in Book 6, it could make them think. But on the other hand, they might be confused as some fanfictions can be really out - there. They'd have to sort through what was based on real knowledge of the wizarding world and what was pure imagination.
Their view could be altered in multiple ways. If the fanfiction presents different moral stances regarding the Deathly Hallows, they might start to reevaluate their own views. For instance, if a fanfic has a character using the Hallows for evil in a way they hadn't thought of, it could make them think about how to better safeguard against such misuses. Also, seeing how modern fans imagine the Hallows' impact on relationships could make them consider the social aspects of magic more deeply.
The wizarding world would likely be plunged into a state of despair. Harry was seen as the 'Chosen One' to defeat Voldemort, so without him, there would be a sense of hopelessness. Voldemort would probably have free rein to impose his dark rule without much opposition at first.