Perhaps in the fanfiction, Naruto sacrifices himself to save the village. He might face an enemy so powerful that the only way to protect the Hidden Leaf and its people is by giving up his own life. This would be in line with Naruto's selfless character. He would rather die than let his friends and the village be destroyed. Another scenario could be that a betrayal within the village leads to Naruto's death. Someone he trusts might set him up, and he falls victim to an ambush or a trap that costs him his life.
In fanfiction, Naruto's young - age death could be due to overusing his chakra. Naruto has a tendency to push himself to the limit. If he overexerts himself in a crucial battle, it could lead to a complete depletion of his chakra, which in turn could cause irreparable damage to his body and ultimately result in his death. It could also be that a curse or some dark magic was placed on him, which slowly drained his life force until he couldn't hold on any longer.
One possible cause could be an illness. Maybe there was a hidden disease that was slowly weakening Naruto, and despite the efforts of the village's medical ninjas, they couldn't save him. Another cause could be a powerful enemy attack that was specifically designed to target Naruto and his unique abilities. This enemy might have found a way to bypass Naruto's defenses and deliver a fatal blow.
In this fanfiction, Naruto might become a Jounin at a young age due to his extraordinary growth. Maybe he has a special training regime with a powerful mentor. His quick rise could also be because of a new and powerful jutsu he discovers. This would make the village elders recognize his strength and promote him early.
One plotline could be that Naruto is taken in by a group of outcasts. He then grows up learning different values from them, and instead of aiming to be Hokage in the traditional sense, he wants to create a new kind of leadership for the outcasts in the village. Another could be that he is found by a wandering ninja who trains him in a more unorthodox way, leading to Naruto having unique skills and a different view of the ninja world.
Another possibility is that she was poisoned. There could be a new type of poison in the 'Vampire Diaries' world that affects vampires in a deadly way. And Caroline, being a vampire, was unable to resist it. Or, it could be related to the ongoing power struggles between different vampire factions. She might have been caught in the middle and killed as a result.
In some fanfictions, the cause could be a curse. There could be an old enemy of the village or someone with a vendetta against Naruto who placed a curse on him. This curse could be slowly draining his life force, causing him to become sick. Or, it could be related to the tailed beast inside him. Maybe there was some imbalance in the chakra of the Nine - Tails that was affecting Naruto's health. It's also possible that he caught a strange and rare disease from an unknown source during one of his travels to a far - off land.
He could marry Hinata. They have a pre - existing relationship in the original series. Hinata has always had feelings for Naruto, so it would be a natural choice. In the fanfiction, their relationship could progress more quickly, leading to an early marriage.
It could be a new villain. Maybe a villain who targets Naruto's loved ones and Naruto is unable to save them. This failure would cause him great heartbreak. For instance, if the villain kidnapped Hinata and Naruto couldn't rescue her in time.
One possible storyline could be that Naruto gets married early due to some political alliance. For example, his marriage could be arranged to strengthen the bond between Konoha and another village. This would bring new challenges as he has to balance his duties as a ninja and his new role as a husband at a young age.
In some fanfics, it could be Anakin's own self - destructive tendencies. He has made so many bad choices in his life that in the end, his own actions lead to his death. For example, if he continues to pursue the dark side without fully understanding its consequences, it could ultimately consume him and lead to his demise.
If Jiraiya made a deal with the village elders that put Naruto in a difficult position without his knowledge. For instance, agreeing to send Naruto on a dangerous mission without his consent just to gain some political advantage for the village. Naruto would be angry at being used and not being informed by Jiraiya.
Naruto's strong sense of justice and his unwavering determination to protect the village could also play a role. He has shown time and again that he would sacrifice anything for Konoha. His moral compass and his courage in the face of danger could be seen as qualities befitting a Jounin, even at a young age.