One possible reason could be a powerful curse placed on him. Maybe an enemy with dark magic managed to put a slow - acting curse that is gradually sapping his life force.
Perhaps in this fanfiction, there is a disease that is unique to ninja world and Naruto has unfortunately contracted it. This disease could be slowly attacking his cells, his chakra network or his very essence, causing him to die slowly as his body and powers gradually fail him.
It could be due to some internal damage from overusing his chakra in a very dangerous way. For example, if he tried to perform an extremely powerful jutsu that his body wasn't fully prepared for, it might have caused irreparable harm to his internal energy system, leading to his slow decline.
One possible plotline could be that Harry is suffering from a mysterious curse. Maybe he got it during a battle with a dark wizard. His friends would then be on a desperate quest to find a cure, traveling to far - off places and consulting with various magical experts.
Perhaps it has to do with a new and mysterious disease. In fanfiction, new and unique diseases can be created. This disease could be something that attacks his unique chakra system, which is different from others. Since his body is so reliant on chakra, if the disease disrupts it enough, it could lead to his death.
Well, it might start with Naruto feeling weak and tired more often. His friends would notice his decline but he would try to hide it at first. As the story goes on, he would start losing his ability to perform jutsus as effectively.
It could also be due to a curse. The world of Lost Girl has many magical elements. Maybe an old, powerful curse was placed on Bo that slowly drained her life force. Despite her attempts to fight it, the curse was too strong, and in the fanfiction, she succumbs to it and dies.
I don't think manga is dying. It still has a huge fan base and new titles are constantly coming out.
In some fanfictions, it could be related to the magical core inside Harry. Maybe his magical core was damaged in some way, and over time, it weakened to the point where his body couldn't sustain itself anymore. Since sleep is a vulnerable state, that's when his body finally gives in. Also, it could be part of a complex plot where the author wants to explore the reactions of other characters to Harry's sudden and unexpected death while sleeping. It can create a lot of drama and angst among the remaining characters in the story.
Maybe Naruto is affected by a dark and ancient curse. This curse was placed on him by an unknown enemy. It starts to take hold gradually, making him weak and sick. He begins to have strange visions and nightmares. His body slowly deteriorates, and he loses his appetite. His friends notice his strange behavior and the physical changes, but Naruto is determined to keep fighting until they find out what's wrong with him.
A new, powerful demigod who is jealous of Percy's fame and power could be the villain. This demigod has discovered a dark spell that they use on Percy, causing him to slowly die. They want to take over Percy's position as the 'hero' among the demigods.
One possibility could be that Naruto has been poisoned by a new and unknown enemy. This poison is slowly eating away at his life force, and he's trying to hide it from his friends and family so as not to worry them. He might be secretly seeking a cure while still fulfilling his duties as Hokage.
One possible reason could be a vampire attack gone wrong. Maybe a rogue vampire targeted Bella and inflicted a near - fatal wound. Another reason might be related to some sort of dark magic or a curse that was unknowingly placed on her. It could also be due to a self - sacrifice situation where she put herself in harm's way to save someone she loves, like Edward or one of her friends.