Other important characters could be the enemies they faced during that era. For instance, the ninjas from other villages they fought against in the wars. These enemies could help showcase the Sannin's combat abilities and strategic thinking. Also, some of their comrades from Konoha, like Kakashi's father Sakumo Hatake, could be included to add more depth to the story.
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.
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.
It could be Jiraiya. He has a really strong bond with Naruto already. He's taught Naruto a lot in the original story too. So in the fanfic, his training could be the most in - depth, covering things like advanced rasengan variations and how to fight against really powerful enemies. His influence on Naruto's character development would also be significant.
Jiraiya would be the most interesting. He has a wild and carefree personality which could be a great contrast to Naruto's initially lonely and neglected state. Jiraiya is also very knowledgeable about the outside world and has many powerful jutsus, especially those related to the toads. He could take Naruto on adventures, teaching him not just about being a ninja but also about life in general. His connection to the prophecy about the 'Child of Prophecy' could also add an interesting layer to the story, as Naruto might be groomed in a different way to fulfill that prophecy under Jiraiya's guidance.
The Cyclops itself is of course the most important character. You could also include its parents if you want to explore its family origin. Another important character could be a wizard or a seer who has knowledge about the Cyclops' past and future.
It's hard to say for sure without reading it, but likely there will be three main characters who are the new sannin. They could be students of the original sannin or completely new characters with no direct connection but similar in their skills and importance.
Obviously Naruto himself. He is the main character and his growth and development during this arc are crucial. Then there's Zabuza and Haku. Their story of being outcasts and their complex relationship is very interesting to explore further. Tazuna also plays an important role as he is the reason Naruto and his team are in the Land of Waves in the first place.
Well, Batman is a must - have. He is the character that can tie all the others together. Then there's Superman, who represents the ultimate power in the League. Green Lantern is also important as he brings the power of the Lantern Corps and his unique willpower. Aquaman is vital as he can connect the League to the underwater world and has his own set of powers. And of course, Martian Manhunter with his shape - shifting and telepathic abilities can add a whole new dimension to the story.
I think Orochimaru would be the most changed. In the original series, he was very much into dark and forbidden jutsu. But if Naruto were to raise him, Naruto's positive influence would likely steer him away from that path. He might become more of a healer or a researcher into non - harmful ninjutsu.
One of the most interesting characters to include is Nagato. His Rinnegan abilities and his connection to the Sage of Six Paths can create some really complex and exciting storylines in a Sennin Naruto fanfic. He could have some sort of interaction with Naruto's Sage Mode, perhaps teaching Naruto something new about the power of the Sages. Additionally, characters like Gaara can be interesting. His relationship with Naruto and his own unique powers can be incorporated into the fanfic in relation to Naruto's Sage Mode journey.