In some cases, it could be that the magic system itself is the antagonist. For example, if there are ancient magical laws or curses that are affecting Harry's judgment and making him act against the Weasleys. The fanfic could then be about how they all fight against the constraints of this magic system to reconcile.
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.
Well, it could severely damage their relationship. The Weasleys, who have been like family to Harry, would be deeply hurt. They might not be as quick to forgive him, and it could create a rift that lasts for a long time.
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.
One possible reason could be a misunderstanding. Maybe Harry overheard something wrong about the Weasleys and in a fit of anger, threw them out. For example, if he thought they were plotting against him when in reality it was a big miscommunication.
It could be a jealous wizard who feels that Harry and his sister are getting too much attention in the wizarding world. This wizard might try to undermine their achievements and even put them in danger.
It could also be some external force that is manipulating Harry into believing the Weasleys are his enemies. For example, a dark wizard who wants to create chaos in the wizarding world by turning Harry against his friends. In this case, that dark wizard would be the main antagonist even though Harry is the one carrying out the revenge actions.
It could be Mr. Weasley. Since he's the head of the family, he might be the one driving the idea of taking the Potter fortune to better provide for his large family. He has the authority within the family and might think he's doing what's best in a tough situation.
It could be a new dark wizard that we haven't seen in the original series. This wizard could be extremely powerful and manipulative, and his main goal is to corrupt Harry. He might be using Harry's fame and his insecurities to draw him into the darkness.
Obviously, the students from Hogwarts would be the main participants. The Gryffindors, Hufflepuffs, Ravenclaws and Slytherins would all be eager to show their skills.
There could be an old wizard who was once shunned by the Potter family. In revenge, he casts a spell on Harry that makes him think his name is 'freak'. This wizard could be lurking in an abandoned part of the wizarding world, waiting for the right moment to strike again or to see Harry completely lose his sense of self.