Well, Naruto himself would be a central character of course. Sasuke could also be involved, as he often has a role in Naruto - related stories. Sakura might also play a part, perhaps she's the one who first discovers some clue about the fourth sannin.
Obviously Naruto himself would be a main character. He could be the one to face the challenges posed by the puppet sannin. Sasuke could also be involved as he has his own powerful skills that could be used against the puppet - related threats.
The main characters are the Sannin - Orochimaru, Jiraiya, and Tsunade. These three are central to the fanfiction as it explores their characters in a darker context.
Obviously, the Sannin themselves - Orochimaru, Jiraiya, and Tsunade are must - includes. But Naruto can also be a great addition as he has connections to Jiraiya. His presence can add a new dynamic as he looks up to the Sannin and tries to learn from their experiences. Another good character could be Sasuke. His dark past and thirst for power can contrast with the Sannin's different outlooks on power and morality.
One idea could be exploring how the fourth sannin, if there was such a concept, might have influenced Naruto's training in a unique way. Maybe they had a special jutsu that they passed on to Naruto secretly.
Well, it could be some of the younger generation characters who have shown great potential. For example, Boruto could be one of them, as he has his own unique set of skills and abilities. Another could be Sarada, with her Sharingan and growing ninja skills. And maybe Mitsuki, with his snake - like abilities.
Perhaps an old enemy of the Sannin resurfaces. This antagonist could see Naruto's attempts to follow in the Sannin's footsteps as a threat. They might target Naruto to get revenge on the Sannin or to prevent a new powerful force from emerging in the ninja world. For example, if there were a group that was once defeated by the combined efforts of the Sannin, they could try to stop Naruto before he becomes as strong as the Sannin were together.
One possible main antagonist could be a rogue ninja who was once an apprentice of Orochimaru and is jealous of Naruto's connection to the Sannin. This ninja might want to steal Naruto's new - found powers related to the Sannin. Another could be a hidden organization that sees Naruto as a threat because of his potential with the Sannin's abilities. They fear he will disrupt their plans for power in the ninja world.
Definitely Hiruzen. He was not only their teacher but also the Hokage at the time. His interactions with the Sannin, whether it was giving them advice or reprimanding them, were crucial. Then there's Dan. His death had a profound impact on Tsunade. In addition, characters like Shikaku Nara or Inoichi Yamanaka could be interesting to include. They were important members of the Leaf village and could have had some interactions with the Sannin, even if they were not as prominent at that time. It would give a more complete picture of the village's ninja population during that era.
One possible plotline could be that Naruto discovers his connection to the Sannin later in his life. This new knowledge gives him access to hidden jutsu or knowledge that the Sannin had. For example, he might learn some unique medical ninjutsu from Tsunade. Another plot could involve Naruto facing enemies who are after the secrets that the Sannin might have passed down to him.
Maybe the Sannin are called upon by a dragon to help save its kind from a dark force. This would involve them traveling to different parts of the Naruto world, facing various challenges and enemies along the way. They might have to learn new skills or unlock hidden powers to be able to help the dragons.