It could be Samehada itself. Since the story is about it being neglected, it makes sense for it to be the main focus. It could have its own 'voice' and thoughts as it tries to deal with the situation.
Maybe a new ninja character. This ninja could be an outcast or someone who is not well - known in the Naruto world. They discover Samehada and see its potential while others overlook it. This new character then becomes the main character as they embark on a journey with Samehada.
It might also be an existing minor character from the Naruto series. For example, a character who has always been in the background but now sees an opportunity with Samehada. They decide to take it under their wing and make it their weapon, which then makes them the central figure in the fanfiction as they face challenges and try to change the perception of Samehada.
Well, it could be about Samehada being overlooked in the Naruto universe. Maybe it's not being used to its full potential. For example, it might be that no one realizes its unique abilities aside from just being a weapon. Or perhaps it's being misused or underutilized by its owners.
It could be a young and ambitious Kiri ninja who has grown up seeing the neglect of his village. He might be determined to change the situation and prove that Kiri is not to be underestimated. Maybe he has a special jutsu or ability that he believes can put Kiri back on the map.
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.
It could be a new character created specifically for the fanfiction. Maybe a young ninja who discovers the neglected dragon and forms a bond with it.
It could be an orphan who has a natural talent for music but has been brought up in a ninja - dominated environment where music is seen as a useless skill. So, they are constantly neglected.
It could be an unknown Uchiha who was overlooked in the main story. Maybe they were born with the Rinnegan but due to some political unrest in their village, they were forced to hide their power and were thus neglected. In the fanfiction, they could come out of hiding and shock everyone with their Rinnegan - based abilities.
It could be a young Saiyan who was sent to the Naruto universe as part of some sort of experiment by the Saiyan race. They are neglected because they seem so different from the ninjas. They would be the main character as they struggle to find their place and purpose in this new world.
A young and talented Iwa ninja who feels left out could be the main character. He or she might be frustrated with how their village is always in the shadow compared to Konoha and wants to change that perception.
Madara Uchiha would also make a great antagonist. Imagine if he was resurrected and saw Naruto with Samehada as a threat to his plans of creating a new world order. He would use his Sharingan and Rinnegan powers to try and take Samehada from Naruto, believing that with it, he could easily defeat all who oppose him and reshape the ninja world according to his will.
It could be someone who wants to steal Samehada for its power. Maybe a rogue ninja who has heard of Samehada's ability to absorb chakra and plans to use it to take over the ninja world.