His decision to leave the Dursleys can make the plot deviate in many ways. In fanfic, it could mean that he doesn't have to wait for the official Hogwarts invitation. He might seek out the wizarding world on his own. This could lead to him encountering different characters or facing different challenges earlier. Also, his departure might make the Dursleys more or less relevant in the overall story. If they are no longer a factor, then new antagonists or plot elements need to be introduced to keep the story interesting.
The story could take a darker turn if he refuses to go back to the Dursleys. Maybe without the protection of the blood magic at the Dursleys' place (even if they were cruel), he becomes more vulnerable to Voldemort's attacks earlier. Or, on the other hand, he could find a new form of protection that was not part of the original story, which could change the power dynamics in the fight against Voldemort.
Well, it could completely disrupt the relationships. For instance, his friendship with Ron and Hermione might be strained as they try to convince him to stay. This is different from the original where they are all united in their Hogwarts experience.
It could go either way. If the fanfic shows Harry in a really positive light, like a super - powerful wizard who is also kind - hearted despite their treatment of him, they might feel a bit guilty. But if the fanfic portrays Harry in a negative way, they might feel justified in their own cruel behavior towards him.
The abuse makes Harry more cautious. He has learned from the Dursleys that the world can be cruel, so he's careful about who he trusts. For example, he might be hesitant to open up to new people he meets at Hogwarts.
In this fanfic, the Dursleys may be depicted as being in over their heads. Vernon Dursley, for example, might be shown as constantly angry and trying to assert his authority in a situation where he has none. Petunia Dursley could be portrayed as being torn between her dislike for magic and her sense of family obligation towards Harry. Dudley might be shown as being bullied or affected in the Muggle world because of Harry's presence in their family.
Harry's character might grow in resilience. The neglect forces him to become self - reliant. He has to fend for himself, which makes him tougher. For example, he learns to cook his own meals and take care of his basic needs without any help. This self - sufficiency builds his confidence over time.
It might show their fear in a more extreme way. Instead of just being dismissive of magic, they could be absolutely terrified of it. For example, every time Harry does something even slightly magical, they could cower in a corner.
Well, in the original story, the Dursleys are cruel to Harry. But in this fanfiction idea, it completely flips the dynamic. Harry would have a much more stable home life, which could mean he doesn't have the same sense of longing and isolation. For example, he might not be as eager to escape to Hogwarts every year.
A fanfic with Cedric and Harry as brothers would change the Hogwarts experience for both of them. They could share a dormitory or have special family - related privileges at school. Their bond as brothers would likely make them more of a united front against the likes of Voldemort. Instead of Harry being the sole focus of Voldemort's attention, Cedric would also be in the crosshairs. This would lead to different strategies and plot developments. The Weasley family might interact with them differently too, as they would be related to Harry in a different way. And the relationship between Harry and Cho Chang (who was Cedric's girlfriend) would be completely reimagined, adding a whole new layer of complexity to the story.
In such fanfiction, the story might change the original relationship by having the Dursleys show kindness early on. For example, they could provide Harry with a warm welcome when he first arrives at their home, instead of making him live in the cupboard. This alteration can lead to a different emotional growth for Harry, as he doesn't have to endure the same level of neglect and abuse. It also changes the dynamic of the entire wizarding world story, as Harry's home life is a big part of his character development.