Definitely the Second Hokage. Another great addition could be the ninja who were in charge of different aspects of the village during his tenure, like the head of the ninja academy. This could show how he influenced the education and training of future generations. Also, a young and ambitious ninja from a minor clan could be a main character. This ninja could look up to the Second Hokage and strive to be like him, while also facing challenges within their own clan.
Obviously, the Second Hokage himself should be a main character. His brother, the First Hokage, could also be included, perhaps in flashbacks. Then there could be some of his students, like Hiruzen Sarutobi. The Uchiha clan members might be important characters too, since they play a significant role in the village's history and the Second Hokage had interactions with them.
The Second Hokage is a must - have main character. Additionally, some of the powerful ninja from other villages could be included. They could be the antagonists, trying to take advantage of the Hidden Leaf Village during a time of change or vulnerability. And of course, the members of his own team when he was a young ninja could be important characters, showing his growth and development as a shinobi.
Obviously, the new Hokage would be one of the main characters. It could be a character we already know like Naruto's son, who has inherited his father's determination and strength.
Sasuke could also be a candidate in some fanfics. He has his own set of powerful abilities and a complex personality. A story could be written where he decides to take on the role of Hokage at a young age to bring about change in the village in his own way, perhaps using his Sharingan and new techniques he develops.
Obviously, Naruto would be one of the main characters as it's about him becoming the sixth Hokage.
I'm not entirely sure without reading the fanfic, but it could be Naruto and his friends like Sasuke and Sakura. Since Naruto is aiming for the Hokage position, his friends might play important roles in the story, either supporting him or having their own arcs related to the perks of the Hokage.
Well, it would likely include Naruto himself as he is central to the Naruto universe. Since it's about the Hidden Hokage, he would probably be a main focus.
Shikamaru is another possibility. He is known for his strategic mind. As Hokage, he could use his intelligence to make wise decisions for the village. He may not have the brute strength of Naruto or Sasuke, but his ability to plan and analyze situations would be invaluable in leading Konoha.
Obviously, Naruto and Ino would be main characters. Their parents, like Minato and Kushina for Naruto, and Inoichi for Ino, could also play important roles. Maybe even their grandparents if there are any backstories to explore.
Sasuke could also be a possibility for the 6th Hokage in the fanfic. He has grown stronger over the years and his unique perspective on the ninja world could bring new ideas to the leadership of the Hidden Leaf. He might start off as a controversial choice but prove himself through his actions in protecting the village and making tough decisions.
It could be a rogue ninja from another village who has a vendetta against Konoha. Maybe he blames the Godaime Hokage for something that happened in the past, like the death of his family during a Konoha - related mission. So, he decides to target the village and the Godaime Hokage in particular.
Perhaps the main antagonist is a spirit or a curse that has been awakened. This curse could be slowly influencing the villagers to turn against Naruto. It could be something that was sealed away long ago and now has broken free due to some mistake or interference. The antagonist could also be a mastermind from another hidden village who is using spies and psychological warfare to undermine Naruto's rule as the Sixth Hokage.