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.
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.
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.
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.
Kakashi Hatake is a popular choice. He has a long history with both Naruto and Sasuke, having trained them. He could be included to offer guidance or add some comedic moments. For example, he could give them some tough training exercises that help them bond more. Another popular character is Sakura Haruno. She has her own complex relationship with both Naruto and Sasuke. Her presence can add some drama, like if she has to choose between helping Naruto or Sasuke in a difficult situation.
Sakura is a very popular character to include. She has a long - standing relationship with both Naruto and Sasuke. Her medical ninjutsu skills can be very useful in various plot situations. Kakashi is also popular. His experience and unique jutsu can add depth to the story. Another popular character is Shikamaru. His intelligence can be a great asset in devising strategies against enemies.
Sakura could be a great addition. She's a strong kunoichi and her medical skills could come in handy in many situations. For example, if Naruto and Sara are on a dangerous mission, Sakura could be there to heal their wounds. Another good character is Kakashi. His wisdom and experience would be valuable, especially when they face tough decisions or difficult enemies.
Iruka could be included. He was Naruto's first teacher and has his own connection to Kakashi. His presence can bring out the different teaching styles between him and Kakashi. Another good character is Jiraiya. His relationship with Naruto as his godfather and his connection to Kakashi through being comrades can add some interesting storylines.
Sasuke could be a great addition. His complex relationship with Naruto and his own power could add a lot of drama. He might be initially suspicious of Guren and her motives, creating interesting conflicts.
Sasuke could be a great addition. He could be a rival to Naruto even at a young age, and Kakashi would have to deal with their constant bickering. His brooding nature would add an interesting dynamic to the story.
Sakura would be a great addition. As Sarada's mother, she could add an interesting dynamic. Also, Boruto could be included. His relationship with Sarada and his own skills could create some engaging storylines. Another good character could be Kakashi. His wisdom and combat experience could be useful in the story.