One possible plotline could be Harry using his newly discovered magical abilities in unexpected ways against the Dursleys. For example, he might make the Dursleys' dinner plates fly around when they are being particularly mean to him.
Obviously, Harry Potter himself is the main character. He is the one with the motivation to fight against the Dursleys due to their years of mistreatment.
In the 'the dursleys abuse harry fanfiction', the abuse can be multi - faceted. Physically, Harry could be pushed around or hit by Vernon. Emotionally, Petunia might constantly compare him unfavorably to Dudley, which is really damaging to Harry's self - esteem. And socially, they might prevent him from having any friends over, making him feel completely alone in the world.
It's possible. In some fanfictions, authors might explore complex motives for the Dursleys' actions. Maybe they could be under some dark magic influence that makes them act out of character and in a more extreme way. However, this goes against the general perception of the Dursleys in the original series, where they are mainly just cruel and neglectful without such extreme murderous intent.
One consequence could be that Harry becomes withdrawn. He's so used to the abuse that he doesn't trust people easily. He might have nightmares often, reliving the moments of abuse. For instance, if Vernon has hit him before, he could have flashbacks in his sleep.
Well, in fanfic, the Dursleys' abuse of Harry can be shown in different ways. They might isolate him from the outside world. For instance, not allowing him to have friends or participate in normal neighborhood activities. They could also starve him, giving him very little food while Dudley gets plenty. This kind of treatment is often explored in detail in fanfiction to show how harsh Harry's life is with the Dursleys.
One way the Dursleys can be good in fanfiction is if they are forced to face the consequences of their actions early on. For instance, they might receive a warning from the wizarding world that if they don't treat Harry well, there will be serious repercussions. So they start to be nicer to him. They could provide him with a proper room, better food, and allow him to have friends over.
In some Harry Potter fanfictions, the Dursleys are depicted as nice. This could be a really interesting take. It might show how they could have been different if they hadn't been so influenced by their dislike for magic. Maybe they could have supported Harry more, or at least been less cruel. For example, a fanfiction could explore a scenario where Vernon Dursleys actually encourages Harry's academic achievements at school, instead of always putting him down.
They would likely face severe punishment from the wizarding world. The Ministry of Magic would not tolerate such blatant abuse of a wizard, especially one as important as Harry.
In some versions, Harry might be saved by his magical abilities emerging in a crucial moment. Maybe he uses magic to make food appear for himself, defying the Dursleys' cruel intentions.