One possible plot could be that the Weasleys might betray Hermione over a powerful magical artifact. Maybe they believe she has become too power - hungry in her pursuit of knowledge about it and they side with those who want to keep it away from her.
In this fanfiction, it could be that the Weasleys betray Hermione because of a misunderstanding. For example, Hermione might be involved in some secret research that the Weasleys misinterpret as something harmful. Their family loyalty to the wizarding world at large makes them think they need to stop Hermione, even though she has good intentions.
Perhaps the Weasleys are forced to betray Hermione due to some dark magic influence on them. It could be that a dark wizard has a hold on the Weasleys and makes them turn against Hermione as she is a key figure in the fight against the dark side. They might give away her secrets or leave her in a dangerous situation.
At first, Hermione would be heartbroken. She has shared so many experiences with the Weasleys. However, her strong will would soon take over. She would probably focus on proving her innocence if that was the cause of the betrayal. She might also try to find a way to reverse whatever situation led to the Weasleys' betrayal. If it was a matter of the Weasleys' own wrong beliefs, she would try to show them the truth, but if they refused to listen, she would distance herself from them while still remaining true to her own values and the greater good.
One possible reason could be a misunderstanding. Maybe there was a miscommunication about something important, like a plan to fight against the Dark Arts. For example, if the Weasleys accidentally left Hermione out of a crucial part of the plan, she might be very angry.
One possible reason could be jealousy. If Harry gets more attention or power than the Weasleys, they might feel envious and turn against him.
Well, it could be a completely innocent and humorous story. Maybe they all got accidentally locked in a magically malfunctioning shower room at Hogwarts. And they have to work together to get out, with Hermione using her intelligence and the Weasleys' unique skills and prank - like ideas. It could be full of banter and light - hearted moments as they figure out a solution.
Rather than focusing on such improper fanfiction, it's better to explore the positive and official stories in the 'Harry Potter' universe, like Hermione's friendship with the Weasleys in the proper context.
In this fanfiction, perhaps Hermione uses her vast knowledge of magic. She might save the Weasleys from a dark wizard's attack. Maybe she discovers a powerful spell to counter a curse aimed at the Weasley family.
Well, in many such fanfictions, the Weasleys might protect Hermione by using their magic skills. For example, they could create protective spells around her when she's in danger. They might also physically stand in her way when she's facing threats, like some dark wizards. And they would use their family influence in the wizarding world to keep her safe from any kind of harm or discrimination.
One possible reason could be jealousy. Maybe one of the Weasleys develops romantic feelings for Hermione and the others don't approve or also have feelings for her. Another reason might be related to her magical abilities. If Hermione has some unique magic or knowledge that the Weasleys think can benefit them or their family in different ways, they could fight over it. For example, her knowledge of ancient spells could be seen as a great asset.
In the fanfiction, it could start with Hermione having a falling - out with her own family due to her magic. The Weasleys, seeing her distress, offer to adopt her. As a part of the Weasley family, Hermione would be involved in all their family traditions. She could bring her intelligence to help with family matters, like helping with George's business by using her logical thinking. And her relationship with Ginny would become even closer, like true sisters, sharing secrets and having girl - talk that they didn't have much opportunity for before.
Power could be a reason. If there's a chance for Ron and Hermione to gain a great deal of power by turning against Harry, some fanfictions might explore this. For instance, a new dark power might offer them something they've always desired, like the ability to reform the wizarding world in their own image. They might think Harry would oppose this, so they betray him to get what they want.