It could also be a new character introduced specifically for the fanfic. Maybe an orphan from a small village who discovers they have the evil eye and then gets drawn into the larger Naruto world, facing challenges and making a name for themselves while dealing with their unique power.
Naruto himself could also be the main character with an evil eye. It would create a huge contrast to his usual positive and determined self. He would have to struggle with the dark influence of the evil eye while still trying to uphold his values of friendship and protecting the village.
The villagers themselves could be the main antagonists. Since Naruto has been neglected, their continued mistrust and hostility towards him could drive his 'evil' turn. They represent the status quo that has failed Naruto, and he might see them as the source of his problems.
Sasuke could also become an antagonist in this scenario. If Naruto turns evil, Sasuke, who has his own sense of justice, would likely oppose Naruto. Their once - strong friendship would be shattered, and they would end up fighting each other, with Sasuke trying to stop Naruto from causing more destruction.
A new character could be created as the main character. This character could be an orphan who discovers they have a single Sharingan eye, and they don't know where it came from. They then embark on a journey to find out about their heritage and the origin of their eye. Along the way, they meet various Naruto characters and learn about the power and responsibility that comes with the Sharingan. They might face challenges from both those who want to steal their eye and those who fear its power.
Obviously, the evil Naruto himself would be the main antagonist. He's the one causing trouble in the Teen Titans' world, so he takes on that role.
I'm not sure exactly who the main characters are without reading the book, but they could be someone who is suspected of having the 'evil eye' and those around them who are affected by this suspicion.
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.