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 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.
There could be some original characters created specifically for the fanfic. For example, a young shinobi from a small village who has a unique power related to nature. This character could have a significant role in the Shinobi Wars, perhaps by forming an unexpected alliance with Naruto and his group. Their journey together, with the new character learning from Naruto and growing, would make them a memorable part of the fanfic.
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.
A female shinobi who defies the traditional gender roles in her clan during the wars would make an interesting main character. She could break barriers and show that women can be just as powerful and important in the shinobi world as men, and her actions could have a significant impact on the outcome of the clan wars.
A civilian who somehow gets involved in the war and discovers they have latent shinobi abilities. They could start as a clumsy and inexperienced character but grow into a powerful force.
One interesting storyline could be about a new and powerful shinobi emerging during the Shinobi Wars and how they interact with Naruto and his friends. This new character might have unique abilities that change the dynamics of the battles.
Naruto would be an obvious choice. He played a major role in the original 4th shinobi war. But in a fanfic, you could explore different aspects of his character, like his internal struggles during the most intense battles.
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.
One idea could be a story where a new, unknown shinobi emerges during the wars with a unique power that changes the balance of power. This shinobi could initially be seen as an enemy but later turn out to be an ally with a noble cause.