The plot could involve Naruto being sent on a mission that requires him to master skills similar to the Sannin. He starts by learning from Kakashi about the different abilities of the Sannin. Then, he travels to different places. In the Land of Waves, he might discover a hidden scroll that has Jiraiya's uncompleted jutsu. He spends time deciphering it and adding his own touch to it. Meanwhile, he also has to deal with enemies who are after this new power he's developing, just like the Sannin had to face many foes during their time.
In a Naruto become a Sannin fanfiction, the story could start with Naruto feeling the need to be stronger than ever. He realizes that the Sannin were some of the most powerful ninja in history. So, he decides to study their ways. He first tries to understand Tsunade's superhuman strength. He spends days and nights training with weights, just like she did in her youth. Then, he delves into Orochimaru's research on forbidden jutsu, not to use them for evil but to understand the limits of ninjutsu. Along the way, he also befriends some of the old comrades of the Sannin and they help him in his journey to become like them.