A plot could be that the cancer is a result of some sort of chakra - related experiment gone wrong in his past. Naruto initially brushes off the symptoms as just being tired from his work. But when it becomes clear it's something more serious, he has to come to terms with his mortality. His friends like Sasuke would be in denial at first, trying to find ways to save him against all odds. The story could end with Naruto's death being a catalyst for the village to make changes in how they handle ninja health and safety.
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.
One possible plotline could be Harry gradually losing his magic as the 'magical cancer' progresses. He might struggle to keep it a secret from his friends, especially Hermione and Ron, who would be desperate to find a cure. Another could be that he starts having strange visions or experiences related to the magical source of this 'cancer' which could lead him on a quest to find its origin in the wizarding world, even as his health deteriorates.
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 some fanfics, it could be a situation where they are trying to save the ninja world from a great disaster. They end up using a powerful jutsu that requires both of their life forces. They know that this jutsu will kill them, but they are willing to sacrifice themselves for the greater good. As they perform the jutsu, they share memories of their past, their rivalry, and their friendship, and then they die together, leaving behind a world that they have saved.
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.
One possible plotline could be Percy finding out he has cancer and having to deal with the reactions of his family. Some might be overly protective, while others might struggle to accept it. Another could be Percy supporting a family member with cancer, showing his growth and change from his more uptight self in the books.
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.
Perhaps the fanfic would show how May's death inspires others to make the most of their lives. Or it could center on a mystery related to her illness and its cause.
One possible plotline could be that Harry is dying from a mysterious dark curse. His friends, like Hermione and Ron, are desperately searching for a cure. They might travel to far - off places, seeking ancient magic or the help of powerful wizards. Another plotline could be that Harry is sacrificing himself for the greater good, perhaps to defeat a new and powerful dark force. His slow death could be a time of self - reflection and him saying goodbye to his loved ones.
It could be about a character who tries to keep the last bit of hope alive in the dying world. Maybe they are on a journey to find a way to revive the world.
Perhaps the plot could involve some sort of magic or curse that is causing Naruto to be in this state. The story might then focus on how the Naruto world's medical ninjas try to figure out what's wrong. They could run all sorts of tests and experiments. And there could be a race against time as Naruto's condition worsens.