Perhaps it's a shinobi who has retired after a long and eventful career. He could be someone who has seen the darker side of the shinobi world and wants to make sure Naruto doesn't go down the wrong path. He might have been a wandering shinobi who finally settles down with Naruto. For example, he could be a shinobi from a different village who has a unique set of skills to teach Naruto.
Well, perhaps the retired shinobi has some secrets from his past. When he adopts Naruto, he starts to pass on these hidden techniques and knowledge. Along the way, Naruto might also help the retired shinobi come to terms with his own past traumas or unfulfilled dreams. As a result, they both grow and Naruto becomes a more well - rounded shinobi under this new mentorship.
Obviously, Naruto is one of the main characters since it's about his retired life as a shinobi. Sakura could also be a main character, perhaps she still works as a medical ninja and has a different relationship with Naruto now that he's retired. Another could be Boruto, Naruto's son, as he would be affected by his father's retired status.
One interesting plotline could be Naruto adjusting to civilian life after retiring. He might struggle with the boredom of not having the constant action and purpose he had as a shinobi. Another could be him mentoring the next generation in a more laid - back way, passing on his wisdom without the pressure of active duty.
It could be an original character created specifically for the fanfic. This character could be a young ninja with a unique power or backstory, like a shinobi who has the ability to control plants in combat.
It could be a rogue ninja who was once part of a minor group during the war but has now risen to power. Maybe they felt overlooked or mistreated during the war and now seek revenge on the major villages.
An older, retired shinobi who is forced to come out of retirement during the shinobi wars. He has old - fashioned techniques but a wealth of experience. He could mentor a group of young shinobi and help them develop new strategies based on his years of combat knowledge.
Naruto himself could be a main character, using his skills and influence to try and stop the clan wars. Another could be a young shinobi from a minor clan who has a special ability and wants to prove their clan's worth. And Sasuke could also be involved, perhaps having his own motives related to the clans.
Perhaps an old organization that was thought to be defeated resurfaces. They were biding their time while Naruto was hokage, and now that he's retired, they believe they can carry out their evil plans. They could be a group with a long - standing vendetta against Konoha and Naruto specifically. They might have new members or new powers that Naruto and his friends have to figure out how to counter.
Well, the shinobi rules in Naruto fanfic can be quite diverse. There's the rule of secrecy regarding jutsu. Shinobi are not supposed to freely share their unique and powerful jutsu with outsiders or enemies. Then there's the code of honor in combat. They should fight fairly, not use underhanded tactics like attacking an unarmed opponent. And of course, they are required to respect the hierarchy within the village's shinobi system.
Obviously, Naruto is one of the main characters as it's about him being a shinobi god. His friends like Sasuke and Sakura are likely to be important as well. They might support Naruto in his new role or have their own story arcs related to Naruto's transformation.