It could be Naruto's father. Maybe he was so focused on other things, like protecting the village, that he didn't pay attention to this family heirloom. He might have thought it was better to keep Naruto away from it for some reason, perhaps fearing that the power of the sword would be too much for him at a young age.
It's possible that the whole family collectively neglected the sword. They might have been through some hard times, like wars or internal disputes, and the sword just got pushed to the side. They might not have realized its importance until Naruto discovers it. Maybe they thought it was just an old relic with no real value, but Naruto will prove them wrong in the fanfiction.
Well, it could be a symbol of Naruto's outsider status in his family. Maybe the sword was once an important heirloom but was neglected because Naruto was different or not fully accepted. It might hold some hidden power that only Naruto can unlock later, tying into his journey of self - discovery and proving himself.
Maybe an outcast ninja. This person has always been on the fringes of the ninja society. Discovering the neglected sword gives them a chance to prove themselves. They could be someone with a lot of untapped potential who uses the sword to rise above the neglect they've faced.
Well, it could be about a sword that has some sort of hidden power. Maybe it was once used by a great shinobi in the Naruto world but has been forgotten over time. The story might follow a character who discovers the sword and tries to unlock its potential while everyone else in the Naruto universe is neglecting it.
The sword could have the ability to cut through any chakra - based attacks. Since Naruto's world is full of ninjas using chakra, this would be a very useful ability. It would allow Naruto to easily defend against and counter his opponents' attacks.
It could be Orochimaru. He's always been interested in power and experimenting. Maybe he sees Naruto as a perfect guinea pig to test some new jutsu that goes horribly wrong and causes Naruto to explode.
It could be Danzo. He has always been a bit of a shadowy figure with his own motives in the Naruto series. He might see Naruto as a threat to his power and influence within the village and engineer a situation where Naruto is executed.
Maybe some of the more conservative village elders. They have always been a bit wary of Naruto because of the Nine - Tails inside him. So they might be behind his capture in the fanfiction.
It could be Madara Uchiha. He was always seeking power and new ways to achieve his goals. Turning Naruto into a weapon would be a great advantage for him in his plans to reshape the ninja world.
It could be a new and powerful villain who has emerged. This villain might have a special ability that catches Naruto off - guard during a fight, leading to his injury.
It could be an old ninja who sees Naruto's potential. This ninja might have left the sword as a way to help Naruto in his future battles.