Well, in some fanfictions, the heartbreak could be a result of a prophecy. The prophecy might state that being with Harry would bring doom to the other person, so they break his heart to save themselves and him. Another plot twist is that the person who breaks Harry's heart is actually trying to protect him from a greater danger. They think that by distancing themselves from Harry, they can keep him out of harm's way. And sometimes, the heartbreak occurs because Harry finds out that the relationship was built on a lie. Maybe the other person was only with him to get close to his magical powers or something related to his fame in the wizarding world.
A plot twist could be that the person who breaks Harry's heart has a secret identity. They could be a spy from an enemy group, but during their time with Harry, they actually fell in love too. However, their loyalty to their original group makes them break Harry's heart. Or perhaps the heartbreak is due to a time - traveling incident. Harry meets someone from the future who has to leave him in order to preserve the timeline. And also, it could be that the one who breaks Harry's heart is actually under a spell that makes them act out of character, and when the spell is broken, they try to come back to Harry but it's too late.
One interesting plot twist could be that the letter is delivered by a completely unexpected character, like a house - elf instead of an owl. This would change the whole dynamic of how Harry receives this important news.
Often, in these fanfictions, a plot twist could be that Naruto's heartbreak is self - inflicted. He might have some hidden secret or past trauma that he doesn't share with his partner, and when it finally comes to light, the relationship falls apart. For instance, he could be hiding the fact that he has a limited lifespan due to some powerful jutsu he used. His partner feels betrayed by the secrecy and leaves him, causing his heartbreak. Another idea is that the love interest is in love with Naruto's shadow, the idea of the hero, rather than the real Naruto. When the real Naruto shows his flaws and insecurities, the love interest loses interest, breaking his heart.
Sometimes, the plot twist is that Stiles is actually in on the kidnapping in a way. Maybe he's trying to protect his friends from a greater danger by faking his own kidnapping. So all the while his friends are searching for him, he's actually on a secret mission of his own, which is revealed later in the story and makes for a really interesting read.
One common plot twist could be that a close friend or ally, who Ichigo thought he could trust completely, turns out to be working for the enemy all along. For example, Orihime might be brainwashed by a powerful Hollow to feed information about Ichigo's whereabouts and plans to the Hollows. Another could be that the Soul Society, which Ichigo has often helped, decides to turn on him due to some false accusations of him being too dangerous. Maybe they believe he is on the verge of losing control of his Hollow powers and decide to eliminate him before he becomes a threat to both the living world and the Soul Society.
One common plot twist could be that Aria survives the shot but loses her memory. This then leads to a whole new set of relationships and self - discovery as she tries to figure out who she is and who would want to harm her. Another could be that the person who shot her is someone she thought was a friend all along, creating a sense of betrayal and mystery.
One common plot twist could be that the kidnapper is someone Allen thought was a friend all along. It adds a sense of betrayal and shock to the story.
In some fanfictions, a plot twist could be that Haruhi's kidnapping is part of a virtual reality game that she unknowingly entered. The 'kidnappers' are just players in the game who are trying to win by capturing her. Another common plot twist is that the kidnapper has a change of heart halfway through. They start to see Haruhi's good qualities and instead of harming her, they end up helping her. And sometimes, it could be that the kidnapping is a test set by a mysterious organization to see if Haruhi is worthy of some special task or knowledge.
One common plot twist could be that the person she trusts most, like her best friend, betrays her for power. Maybe her friend is offered a high - ranking position in a magical guild or something and has to sacrifice Lucy to get it. Another could be that a love interest betrays her. He might be secretly working for an enemy and using Lucy to get information about her family or her magic.
One common plot twist could be that Hinata finds out she was actually set up. Maybe someone was jealous of her relationship and made it look like her partner cheated. This would add a layer of mystery and give Hinata a chance to fight for her relationship.
One common plot twist is that his family actually has a noble reason for abandoning him. Maybe they were trying to protect him from a greater evil that they knew was coming for him. Another is that Harry discovers he has a hidden power that his family was afraid of, which led to the abandonment. For example, he could have a power that is considered dark but is actually something that can be used for good.