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.
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.
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.
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.
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 an original character. For example, a young ninja from a small, unassuming clan that has been overlooked in the grand scheme of the war. But as the war progresses, they show extraordinary skills and courage, becoming a key figure in the battles.
Sasuke could also be the main character. His quest for power and revenge could be further explored in the context of the war. He might find himself in situations where he has to choose between his personal goals and the greater good of his village and the shinobi world.
Rock Lee could be an interesting main character for a Third Shinobi War fanfiction. Since he doesn't rely on ninjutsu, his unique fighting style could be a great asset during the war. We could see him face off against powerful ninjutsu users from other villages and prove that taijutsu is just as effective. Also, his unwavering determination could inspire his comrades during the dark times of the war.