It could also be that in the fanfiction world, Hermione has been corrupted by power or ambition. She might see Harry as an obstacle to her achieving greater things in the wizarding world. So, she betrays him to gain more power, access to restricted knowledge, or to climb the social ladder within the magical community.
In some fanfictions, Hermione might betray Harry due to a misunderstanding. For example, if there were false rumors spread about Harry being involved in dark magic or having a secret, evil plan. Hermione, being a stickler for the rules and the greater good in the original series, might be misled into believing these rumors and thus betray Harry.
Well, for Harry, it would be a huge emotional blow. He trusts Hermione deeply, so her betrayal could make him question his own judgment. It might also make him more isolated as he loses one of his closest allies. For the wizarding world, if Hermione was on the wrong side, it could lead to chaos as she is very intelligent and powerful. Her knowledge of spells and magic could be used against the good side.
Perhaps the collar is a cursed object that Hermione and Harry need to deal with. It could be causing strange behavior among the students. For instance, those who touch it start acting like animals. Hermione would use her extensive knowledge of magic to research a solution, while Harry would be more hands - on, trying to keep the affected students safe.
Another possibility is a miscommunication or false accusation within the fanfiction world. Someone could spread lies about Harry among the Weasleys, leading them to believe he has done something terrible and thus turning against him. But this is a far cry from the deep bond they share in the official 'Harry Potter' story.
In some fanfictions, Harry could be betrayed by friends due to jealousy. For example, if a friend is jealous of Harry's fame as the 'Chosen One'. Another reason might be a misunderstanding. Maybe Harry is wrongly suspected of using dark magic, and his friends, without proper investigation, turn against him.
In certain fanfictions, Dumbledore's secrets are the root of the perceived betrayal. For instance, his secrets about the Deathly Hallows. If Harry discovers that Dumbledore had known more about the Hallows all along and didn't share this knowledge, which could have changed Harry's journey and decisions, it could be written as a betrayal. Harry may feel that he was used and misled by Dumbledore, and thus the idea of betrayal emerges in these fan - created stories.
Well, it could be that in these fanfictions, a dark influence overcomes Hermione and Ron. They might be manipulated by a powerful dark wizard who makes them believe that betraying Harry is the right thing to do. It could also be a miscommunication situation where they think they are helping but end up betraying Harry. For instance, they might be tricked into believing that Harry has turned bad and needs to be stopped.
In some fanfictions, it might be because of a love triangle gone wrong. Suppose Ron had unrequited feelings for Hermione and thought she favored Harry too much. And Harry, perhaps misinterpreting Hermione's actions in a crucial situation, joined Ron in betraying her. This would be a complex and emotional reason for the betrayal.
One possibility is that the truth potion gets misused. For example, an enemy of Harry and Hermione might get their hands on it and try to use it on them. But Hermione, being the brilliant witch she is, could find a way to turn the situation around. She might use her knowledge and quick thinking to make the enemy believe false information while under the influence of the potion. And this could be a crucial part in their fight against the dark forces.
There are many great fanfictions about Harry Potter and Hermione Granger. Some focus on their adventures during the Hogwarts years that were not shown in the original books. For example, there could be a story about them solving a mystery in the Forbidden Forest on their own. These fanfictions often explore their friendship in more depth and show how they rely on each other's skills and knowledge.
One possible plot could be them dealing with the dark forces that still lurk in the wizarding world even after Voldemort's defeat. Their combined skills and knowledge as a married couple make them a powerful force against any new threats.