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 a great choice. He has powerful abilities and a complex past. His experience as an avenger and then his redemption arc gives him a lot of depth. He has also learned from Naruto and has a different perspective on leadership. He could bring a more strict and strategic approach to being Hokage compared to Naruto.
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.
A group of corrupt politicians within the village itself could be the main antagonists. They might be secretly plotting to undermine Naruto's authority as the 5th Hokage in order to gain more power for themselves. They could be making deals with other villages or ninja groups that are not in the best interest of Konoha, and Naruto has to expose and stop their schemes.
It could be some of the old enemies who were thought to be defeated but managed to come back. For example, Orochimaru's former followers who still hold a grudge against Naruto and Konoha. They might see Naruto's retirement as an opportunity to strike.
Perhaps a secret organization that had been hidden for centuries and didn't want the new order that Konoha represented. They could have dark magic or forbidden jutsu at their disposal and use it to try and stop the First Hokage from achieving his goals. This would force the First Hokage to not only rely on his physical strength and known ninja techniques but also to find ways to counter the unknown and mysterious powers of this secret organization.
A group of rogue ninjas who have made the island their base could be antagonists. They might see Naruto as a threat to their operations on the island.
It could be some rogue ninjas who were against the peace treaties after the war. They might form a group and try to disrupt the newly - established peace in the ninja world.
A group of traditionalists in Akeno's world could be antagonists. They might not approve of the idea of a harem or of an outsider like Naruto having so much influence. So they could try to break up the harem and drive Naruto out of their world by using political means, like spreading false rumors about Naruto and his friends.
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.