Even Naruto himself could be a subject of this 'evil Hokage' concept in fanfiction. Maybe he is possessed by some dark entity or has a major change of heart due to some extreme event. This would completely change his character and turn him into an evil Hokage, which would be a very interesting and unexpected twist.
Naruto himself could be a potential character, perhaps in a mentoring role to the 6th Hokage. Sasuke could also be there, maybe as an ally or a rival in some aspects of the story.
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.
Perhaps a powerful and corrupt merchant group could be the antagonists. They are secretly trying to take over the village's economy and resources. They use bribes and blackmail to get their way. The Sixth Hokage discovers their plot and has to stop them without causing too much chaos in the village's economic system. They may also have some ninja mercenaries working for them.
Sasuke could be an alternative. He has strong ninja skills and a deep understanding of the ninja world. His experiences have made him grow into a very capable person, and his Sharingan gives him an edge in combat and strategic planning.
A possible plot is that the evil Hokage has been corrupted by power. They start to impose extremely harsh rules on the villagers. Any form of dissent is brutally punished. They also might be using the village's resources for their own hedonistic desires, like building huge palaces for themselves while the common villagers suffer from poverty and lack of basic necessities.
The other ninja clans' leaders would be important characters too. For instance, the Uchiha clan leader (if the story is set at a time when the Uchiha were still in the village) could play a role in the power dynamics related to the fifth Hokage. And of course, the Hokage's personal friends and mentors from their younger days.
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.
Naruto himself could be a likely candidate. Given his strength, leadership skills, and his dream of uniting the ninja world, he would make a great 6th Hokage.
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 the First Hokage, Hashirama. Naruto would be amazed by his powerful Wood - Style jutsu and his ability to create the village. He could learn about the original ideals behind the Hidden Leaf from Hashirama.