It could be a new character, like a young orphan who discovers their Smoke Release ability and then tries to find their place in the ninja world. They might face discrimination at first because of their unknown origin and unique power, but eventually prove themselves.
Maybe an existing minor character from the Naruto series, such as Ebisu. Imagine if he had a hidden Smoke Release ability that he had to develop in order to face a new threat. His comical nature could add an interesting twist to the story as he tries to be serious about mastering this new power.
It could be an original character. A young ninja from a small, unremarkable clan who discovers they have the Steel Release ability. This character would then have to navigate the complex world of the Naruto universe, facing both the admiration and envy of other ninjas.
It could be an old hermit who has been living in seclusion. This hermit has mastered crystal release and sees potential in Naruto. He decides to pass on his knowledge to Naruto, seeing him as a key to bringing peace to the ninja world.
A fanfic could focus on a group of ninjas who specialize in Smoke Release and form a special unit. They are sent on secret missions where their ability to blend in with the smoke and move undetected is crucial. Their relationships and how they work together could be explored, as well as the challenges they face against opponents with different elemental abilities.
Maybe a mysterious traveler from a far - off land. This traveler has mastered many unique jutsus including Dust Release. He crosses paths with Naruto during one of Naruto's solo adventures and takes him under his wing after seeing Naruto's eagerness to learn new techniques.
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.