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.
In some 'harry fights the dursleys fanfiction', Harry could use his knowledge of the wizarding world to expose the Dursleys' wrongdoings to the magical community. He might send a Patronus to the Ministry of Magic with evidence of how the Dursleys have mistreated him. This would not only be a form of fighting back but also a way to seek justice within the context of the wizarding world.
One possible plotline could be that the Dursleys start to show their love by secretly protecting Harry from the wizarding world's threats. Maybe they learn about a dark wizard targeting Harry and use their Muggle ways to keep him safe without him even realizing it.
Well, in some fanfictions, Harry might control the Dursleys by making them understand the wizarding world. He could show them magical things and, as a result, they start to respect him and his abilities. This could lead to a whole new dynamic in the household, where the Dursleys are no longer mean but rather in awe of Harry and his magic.
One interesting plotline could be Harry showing early signs of magic that the Dursleys try to suppress. For example, he might accidentally make Dudley's toys float when he's angry or frustrated.
One possible plotline could be Harry finding a new family among the magical creatures. For example, he might be taken in by a friendly centaur herd who teach him their ways of magic and survival. Another could be Harry being found by a secret group of wizards who are against the Dursleys' cruel treatment and they help him develop his magic in a more positive and nurturing environment. And there could also be a plot where Harry discovers a hidden magical power within himself as a result of the abandonment, like the ability to communicate with the spirits of Hogwarts, which then guides him on a new path to face Voldemort.
In certain fanfictions of this type, Harry could be found by a powerful but reclusive wizard. This wizard decides to mentor Harry in the ways of ancient and lost magics. Harry then uses this knowledge to not only become a very powerful wizard but also to change the wizarding world's view on things like blood purity. He may also go on a journey to find out why the Dursleys abandoned him and in the process, uncover some dark secrets about his family that had been long buried.
Another plotline might involve Harry making friends outside of Hogwarts who help him deal with the abuse at home. These friends could be from the wizarding community or even muggles with special abilities. They could work together to expose the Dursleys' behavior to the wizarding world, leading to Harry being removed from their care.
One interesting plotline could be that Harry discovers a new, powerful magic that gives him an edge in the fight against Voldemort. For example, he might find a way to combine different spells in an unexpected way. Another could be that Voldemort has a secret weakness that Harry uncovers through his friendships. Maybe one of his friends has a unique connection to an ancient magic that can be used against Voldemort.
One possible plotline could be Harry discovering a hidden magical ability that helps him escape. For example, he might find out he can teleport short distances, which he uses to get away from the Dursleys' house when they are being particularly cruel. Another could be that he gets help from an unexpected magical creature that has been secretly observing him all along. Maybe a house - elf that takes pity on him and shows him a secret passage out of the house.
One possible plotline could be Harry finding a way to break free from the chains and use his magic to confront the Dursleys about their mistreatment. Another could be the Dursleys realizing the error of their ways after chaining Harry up and having a change of heart. Or perhaps there are magical forces at work that lead to a series of adventures while Harry is still chained, like dealing with strange creatures that are attracted to the situation.
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.