One common plot twist is that Ron and Hermione are actually under the Imperius Curse. So they don't have control over their actions when they betray Harry. Another could be that they are trying to protect Harry in a very wrong way. They might think that by distancing themselves from him or even giving false information about him, they can keep him safe from a greater threat.
A really interesting plot twist is when the magical world is split into two factions and Ron and Hermione end up on the opposite side of Harry by accident. They might believe in a different ideology that seems to conflict with Harry's actions. For example, if there is a debate about how to deal with the dark forces, they might choose a more peaceful but in the end wrong approach which means they have to oppose Harry. This creates a complex web of relationships and motives within the fanfiction.
Jealousy is a big one. Ron often feels like he's in Harry and Hermione's shadow. He might see their achievements and feel left out or unimportant, which could lead to his betrayal.
One common plot twist is that they are secretly dating to avoid the attention of the wizarding media. Since they are both friends of Harry Potter, they might want to keep their relationship private from the prying eyes of the press.
One common theme is trust. When Ron betrays Hermione, it often shatters the trust they've built. Another is heartbreak. Hermione usually experiences a great deal of pain. For example, if Ron cheats on her, it breaks her heart. And there's also the theme of forgiveness. Sometimes the story is about whether Hermione can forgive Ron for his betrayal.
Hermione might seek revenge. Given her intelligence and magical abilities, she could use her skills to get back at Harry and Ron for their betrayal. This could lead to a big rift in their friendship and even endanger their common goals in the wizarding world. She might also side with other characters who are against Harry and Ron, changing the whole dynamic of the story.
Well, in these fanfictions, a common plot twist is that the relationship between Ron and Lavender turns out to be a big misunderstanding. Maybe Ron was under a spell or something, and when he realizes it, he has to grovel to get Hermione back. Another plot twist could be that Hermione, in her heartbreak, discovers a new love interest who is much better for her than Ron ever was. This new character could be from a different house or even a different magical community. Also, sometimes the plot twist is that Lavender herself pushes Ron back to Hermione because she realizes they are truly meant to be together.
At first, Harry might be confused. He would try to figure out what went wrong. He might search his memory for any signs that could have led to this betrayal. Then, he would likely experience a range of emotions from sadness to rage. Depending on the nature of the betrayal, he could either try to win them back by showing them the truth or decide that they are no longer worthy of his friendship and focus on his own path, maybe even leaving the wizarding world for a while to clear his head.
In some fanfictions, it could be a misunderstanding that spirals out of control. Hermione and Ron misinterpret one of Harry's actions. For instance, Harry might be trying to protect them in a secret way, but it looks like he's up to something bad. Their lack of trust then grows, and they end up betraying Harry, perhaps by not coming to his aid when he really needs them, which could have serious consequences for Harry.
It could be that a prophecy reveals something that makes Ron and Hermione think Harry is a danger to the wizarding world. They might decide to betray him in order to protect the greater good. However, as time goes on, they could realize their mistake. But by then, the damage to Harry's trust would already be done, and he would have a hard time forgiving them.
A fanfiction that has a unique plot twist is one where Ron and Hermione accidentally switch bodies. This leads to all sorts of comical and also deep - thinking situations as they have to understand each other's perspectives in a very literal way. They also have to keep it a secret from Harry and the others at Hogwarts.
One common plot twist is that Hermione actually doesn't have feelings for either of them at first. She's more focused on her studies or some other important mission. But as the fight between Ron and Draco escalates, she starts to see them in a different light. Maybe she realizes that Ron's jealousy is a sign of his deeper feelings for her, or she discovers a hidden, more noble side to Draco.