It could be that there was some sort of magical accident caused by a Weasley invention that severely affected Harry. He might have suffered physical or mental harm, and after failed attempts at amicable resolution, he sees no other option but to sue the Weasleys in the hope of getting compensation and justice.
Perhaps Harry sues the Weasleys because he believes they misused some of his fame for their own financial gain. For example, they might have used his name to promote their joke shop without his permission. And Harry, being fed up with this exploitation, decides to sue them to set things straight.
Another possibility is that the wizarding legal system or a corrupt official within it could be the main antagonist. They might be trying to make things difficult for Harry, or perhaps they are in cahoots with the Weasleys and are trying to cover up the Weasleys' misdeeds. This would add an interesting layer of complexity to the fanfic.
In this fanfic, Harry could be suing the Ministry over their education policies. For example, Hogwarts might be facing some unfair regulations that are harming the students' learning experience. Harry, having a strong sense of justice, takes on the Ministry. He might gather support from other students, teachers, and even some of the wizarding families who are also affected by these policies. He would then present his case in a wizarding court, which could be full of all sorts of magical procedures and challenges.
Maybe Harry Potter sues the Ministry for false accusations or for not taking proper actions against dark forces. It could also be about some new laws the Ministry made that directly harm Harry or his friends.
Maybe Harry sues the Daily Prophet for spreading false rumors about him, like making up stories that he's secretly working for Voldemort again. It could be that these false reports are ruining his reputation and causing problems in his relationships at Hogwarts and in the wizarding community.
One possibility is that Harry has been betrayed in some way by the Weasleys. Maybe he discovers they've been using him for their own gain, like getting access to his fame or fortune. So, in a fit of anger and disappointment, he throws them out.
Perhaps in this fanfic, the Weasleys have some actions that conflict with Harry and Hermione's values. For example, if the Weasleys were to make a deal with a dark wizard for the sake of their family's safety and Harry and Hermione found out. This could lead to a rift and hatred. Also, it could be related to a love triangle situation in a very convoluted way. Maybe one of the Weasleys unknowingly gets in the way of Harry and Hermione's relationship, or vice versa, which causes animosity.
Maybe in the fanfic, there was a huge misunderstanding. For example, perhaps the Weasleys were wrongly accused of something that endangered Harry's life or the lives of his loved ones. It could also be that there was some sort of dark magic at play that made Harry believe the Weasleys were his enemies.
Maybe in that fanfic, there are misunderstandings between Harry and the Weasleys. For example, they might have had an argument over something like magic use or family values that led Harry to dislike them.
One possible reason could be a misunderstanding. Maybe some dark magic was involved that made Harry believe the Weasleys were acting against him or the wizarding world in a way that seemed illegal. For example, if there was a spell that distorted his perception of their actions.
One possible reason could be that Harry has post - war trauma. He might have seen so much death and destruction during the war that he doesn't want to bring his darkness and pain to the Weasleys. He may think that staying away is a way to protect them from his own emotional turmoil.