Maybe a new, powerful villain that we haven't seen in the original series. This villain could be from a far - off land and have heard of Naruto's reputation. Wanting his power, they find a way to steal his body.
Another possibility is that it could be a member of a secret organization within the Naruto world that has been studying ways to take over others' bodies for their own gain. They target Naruto because of his importance and power.
It could be Orochimaru. He's always been interested in obtaining powerful bodies for his experiments. Naruto's body, with the Nine - Tails' power, would be a great target for him.
It could be a disgruntled house - elf. Maybe one that was mistreated by its wizarding family and decided to steal money as revenge. They could use their magic to get into places unnoticed and take the money.
It could be an old enemy of the Hyuga clan. Since Hinata is a part of the Hyuga, someone with a vendetta against the clan might target her.
It could be a jealous classmate. Maybe someone who is envious of Hermione's intelligence and always being at the top of the class.
It could be Morgana. She has been known to be an antagonist and has a vendetta against Camelot and those associated with it. Poisoning Merlin would be a way to weaken Arthur's most powerful ally.
In such a fanfiction, perhaps an enemy with powerful jutsu manages to steal Naruto's body. This could lead to chaos in the Hidden Leaf Village as they might think Naruto has turned against them. His friends like Sasuke and Sakura would be extremely confused and start to investigate.
It could be a powerful Hollows or Arrancar. Since they are often the antagonists in the Bleach universe, they might see Naruto as an intruder or a threat to their plans. Maybe a powerful Hollow with a thirst for powerful souls targets Naruto, thinking his unique chakra would be a great addition to its power.
It could be Orochimaru. He's always up to no good and might want to frame Naruto to create chaos in the village. He has the intelligence and the resources to plant false evidence.
Veterans themselves might be interested. They can relate to the experiences in the stories, and it could be a way for them to connect with others who had similar, yet unheard, experiences during war. It can also be a form of catharsis for them.
Authors could also be involved. An author might steal the true story of a family or a community. Let's say a small community has a unique cultural story that has been passed down. An author could take that story, make some minor changes, and publish it as their own, without respecting the origin of the true story.
It could be a rogue ninja from a neighboring village. Maybe they were hired by a shadowy organization that wanted to disrupt the balance of power. This ninja might have been specifically targeting Naruto because of his status as a hero and a symbol of hope for the Hidden Leaf Village.