It could also be a side - effect of some powerful but experimental magic he was attempting. Harry is known for his bravery and might have been trying out a new spell that was too much for his body to handle. This could have caused some sort of internal damage, leading to him coughing up blood. For example, if he was trying to tap into a very ancient and powerful magic source without proper preparation, it could have backfired on him.
Well, it could be related to some sort of poison. Perhaps he was poisoned by an enemy who managed to slip something into his food or drink. The poison could be slowly corroding his insides and making him cough up blood as a symptom. It might also be part of a plot by a new villain in the fanfiction world who is trying to weaken Harry in a more insidious way than the usual dark wizards.
It could be a new dark wizard. There are always new threats emerging in the fanfiction world. This new dark wizard might have a vendetta against Harry and uses a special spell or potion to make him cough up blood.
One reason could be to gain power. In fanfiction, the author might want to show Harry in a situation where he needs a significant boost in power and blood magic seems like a way to achieve that. Another reason could be for revenge. If he's been wronged greatly, he might turn to blood magic as a more extreme form of retaliation.
Well, it could be a result of a dark spell cast on him by a Death Eater. Maybe they found a new and insidious way to harm him without leaving an obvious trace.
One reason could be that the official Harry Potter canon is so extensive and detailed that it overshadows fanfiction. The original story has such a rich world, characters, and plotlines that it becomes the main focus for many fans, leaving less room for fan - created stories.
One reason could be safety. Some fanfiction authors might think that Hogwarts is too dangerous for Harry, so they make him homeschooled to keep him out of harm's way.
Some fanfiction writers may have a different vision for Harry Potter's character. They might see him as more of a diplomat or a healer within the wizarding world, rather than a fighter. So, they write stories that reflect this different view of the character, where he uses his wits and other skills rather than engaging in physical or magical combat.
One reason could be to explore the darker side of human nature. Even though Harry is a hero in the original series, fanfiction writers might want to see how he would be affected if he was pushed to his limits. Another reason is to create a more complex and controversial storyline. It grabs readers' attention as it goes against the normal perception of Harry.
One reason could be overexertion of his chakra. In many fanfictions, Naruto might push his chakra limits while using a powerful new jutsu he's created or trying to master. This overextension can cause internal damage, leading to coughing up blood as his body struggles to cope. Another possibility is that he's been poisoned. Some fanfic writers might introduce a new villain who uses a slow - acting poison on Naruto, which gradually weakens him and causes such symptoms.
Well, one reason could be that he was poisoned. In the world of Harry Potter, there are many poisonous substances. A fanfic writer might create a scenario where Harry is unknowingly poisoned, and this causes him to throw up blood as his body tries to fight off the toxin. Another possibility is that he has some sort of internal injury from a fight. If he got hit by a powerful spell or a physical attack during a battle, it could lead to internal bleeding which results in throwing up blood.
One reason could be a reaction to the Ministry of Magic's mismanagement. In the original story, the Ministry often made mistakes and had corrupt elements. In fanfiction, this could lead to Harry becoming an anarchist as a way to oppose such a flawed system.