Perhaps in this fanfiction, on his birthday, Naruto is targeted by a powerful curse. This curse could be placed on him by a dark sorcerer who wants to end the hope that Naruto represents. As the day progresses, the curse slowly drains his chakra and eventually his life, while his friends are desperately trying to find a way to break it.
In this fan - made story, it could be that Naruto is facing an internal struggle on his birthday. He might be dealing with some dark side of his power that he has been suppressing for a long time. Due to some unforeseen circumstances on his birthday, this dark side takes over, and in the ensuing battle within himself, he loses his life as his body and mind are unable to withstand the strain.
A plot could also involve Naruto's friends finally realizing they forgot his birthday at the last minute. They then scramble to throw together a party. However, Naruto has already left Konoha feeling dejected. His friends then have to search for him all over the place, and when they find him, they have a heartfelt apology and celebration that not only includes a party but also a promise to never forget again.
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.
Maybe Ichigo's death occurs due to a curse that he unknowingly activated. In the fanfic, this could lead to a whole new set of rules in the Bleach universe. His friends might then have to search for a way to break the curse and resurrect him. It could involve traveling to unknown realms or making deals with mysterious entities.
In a fanfiction with Naruto dying from cancer, the story might focus on how the ninja world reacts. His allies from different villages might come together to try and find a cure. However, Naruto, knowing the slim chances, might start to prepare for his death. He could pass on his teachings and the will of fire to the next generation in a more urgent and poignant way. His death could also lead to a shift in the power dynamics among the villages, as he was a unifying figure.
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.
One possible plotline could be that they are reborn into a different world altogether. Maybe a world where ninjas don't exist, and they have to figure out their new identities and how to get back to each other. They might have some hazy memories of their past lives which gradually come back as they grow older.
In this kind of fanfiction, Naruto might feel extremely sad and lonely on his birthday. Maybe his friends forget his birthday, or he is rejected from a special event or by someone he cares about. It could lead to him reflecting on his relationships and his place in the village.
In this fanfiction, Naruto could be visited by the spirit of his father, Minato. Minato could offer some words of wisdom and comfort to Naruto on his birthday. They might have a heart - to - heart conversation about Naruto's growth as a ninja and his future in the village.
One possible scenario could be in a great battle against an extremely powerful enemy. Maybe Naruto uses up all his chakra trying to protect the village from a huge threat like a new and terrifying Akatsuki-like organization. His body just gives out from the overexertion.
Maybe Percy is cursed by a powerful god or goddess who is angry with him. The curse causes his life force to slowly ebb away. His demigod powers start to fade too. His friends notice strange things like his sword not being as effective in battle and his ability to control water weakening. They then go on a quest to find the source of the curse and break it.