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.
One possible plot could be Naruto facing the challenges of rebuilding the village after a major war. He'd have to deal with political intrigue from within the village, like some clans trying to gain more power. Also, he might focus on training the next generation of ninja to be stronger and more united.
Yes, there are likely many fanfics with that theme. The Naruto fandom is huge and creative, so it's common to find fan - made stories exploring different 'what - if' scenarios, like Naruto becoming the fifth Hokage. You can look for them on popular fanfic websites.
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.
Perhaps an old organization that was thought to be defeated resurfaces. They were biding their time while Naruto was hokage, and now that he's retired, they believe they can carry out their evil plans. They could be a group with a long - standing vendetta against Konoha and Naruto specifically. They might have new members or new powers that Naruto and his friends have to figure out how to counter.
It could be a rogue ninja who was once a part of the Akatsuki but managed to escape before their downfall. He's been biding his time, growing stronger, and now he wants to take revenge on Naruto and the Hidden Leaf for what happened to the Akatsuki.
A group of rogue ninjas from other clans who oppose the idea of the Hidden Leaf Village. They see the Shodai Hokage's leadership as a threat to their own power and freedom. They might team up and use underhanded tactics like assassinations, sabotage of the village's resources, and spreading false rumors to try and undermine the Shodai Hokage's authority. They could also form alliances with other enemies of the village, making them a formidable and complex antagonist group.
It could be someone who wants to expose Naruto's secret. Maybe a rogue ninja who wants to use this information to create chaos in the village by making the villagers doubt Naruto's intentions.
The Godaime Hokage herself is of course very interesting. Her unique abilities and leadership style make her stand out. Then there's often a new generation of ninjas who interact with her. They bring fresh perspectives and challenges. Also, some fanfictions might introduce new villains specifically designed to oppose her, and these villains can be really interesting with their own motives and powers.
He is likely to be more mature. As the Hokage, he has to make tough decisions. He can't be as impulsive as he was when he was younger. His leadership skills are on full display, and he has a great sense of responsibility towards the village and its people.
It could be a rogue ninja who was once a part of Konoha but was banished for unethical experiments. Now, with a thirst for revenge against Naruto and the village, he returns as a powerful and unpredictable foe.