Another option could be Rock Lee. He's always been about physical strength and unique fighting styles. If he were to have blade arms, it would completely change his fighting dynamic. He could incorporate his existing taijutsu skills with the new blade arm abilities, and it would be interesting to see how he trains to master this new power.
Another possibility is a powerful shinobi from a rival village who believes that the black blade should belong to their village. They see Naruto as an unworthy wielder and will stop at nothing to take the blade from him, leading to a series of battles and political tensions between the villages.
A foreign ninja from a far - off land who has heard tales of the Totsuka Blade and travels to the Naruto world to seek it out. He could have unique abilities that interact in strange ways with the blade.
One idea could be a story where the character with blade arms has to protect a hidden village from a new and powerful enemy. Maybe they use their unique blade arms to fend off attacks and discover a secret about their origin in the process.
Naruto himself could be the main character in a Shinju fanfic. He has always been about finding new ways to bring peace and understanding. His connection to the nine - tails could play an interesting role if he were to interact with the Shinju. Maybe he could try to convince the Shinju that the ninja world has changed and it doesn't need to be a source of fear and destruction.
Sasuke Uchiha could be a great choice as the main character. He already has a complex backstory and a lot of room for character development. His journey from seeking revenge to finding his own path in the Naruto series can be further explored in a fanfic.
It could be an original character. Maybe a young ninja who discovers their forging abilities by accident. They might come from a small, unassuming clan that has kept the forging secrets hidden for generations. This character would then have to navigate the Naruto world while keeping their newfound power a secret at first, until they realize they can use it to make a difference.
Perhaps Sasuke could be the main character in a different take on a 'hitman naruto fanfic'. He already has his own vendetta and his skills as a ninja could easily be translated into those of a hitman. He has the Sharingan, which gives him an advantage in combat and tracking, making him a formidable assassin - like figure.
It could be a new character that you create specifically for the fanfic. This character could be an outcast from their village because of their strange Koton abilities. They then have to prove themselves and their powers to the others.
Sasuke could also be a main character in such a fanfic. Given his quest for power and his complex personality, his encounter with the Shinigami could lead to some really interesting character development. He might be tempted by the Shinigami's power or be forced to make difficult choices that would change his path.
Rock Lee could be an excellent main character for such a fanfic. Since he already focuses on physical combat in the form of taijutsu, wushu would be a natural extension for him. He could be sent on a journey to learn the true essence of wushu and bring that knowledge back to Konoha to revolutionize the way they think about physical combat.