Well, a great plot could be centered around the 5th Hokage's relationships. For instance, how she manages to balance her duties with her friendships and family (if any in the fanfiction). It could also be about her making alliances with other villages in a very different way than before. Maybe she uses her charm and intelligence to broker deals that no one thought possible. And she could be involved in a mystery that threatens the safety of the village, and she has to use her wits and skills to solve it.
One plot idea could be exploring the 5th Hokage's early days in the role. Maybe she had to deal with internal political unrest in the village. Another could be her training a new generation of shinobi with a unique teaching style. Or perhaps a story where she has to face a long - lost enemy from her past that resurfaces to challenge her leadership.
One plot idea could be Naruto facing a new threat from an old enemy who has found a way to return stronger. For example, Orochimaru could have created a new form of jutsu that allows him to possess multiple shinobi at once and Naruto has to figure out how to stop him without harming the innocent. Another idea is Naruto dealing with political unrest within the Hidden Leaf. Some clans might not be happy with his new policies regarding ninja training and he has to find a way to keep the peace. A third idea could be Naruto going on a diplomatic mission to a far - off land where they have a very different view on ninjas and he has to convince them that the Hidden Leaf means no harm.
One idea could be Naruto having to deal with a new and powerful enemy while still adjusting to his role as the 5th Hokage. Maybe this enemy has a unique jutsu that can disrupt the village's chakra network. Another idea is Naruto using his new position to reform the shinobi education system in the village. He could introduce new training methods that focus on teamwork and moral values as well as combat skills.
Perhaps the 8th Hokage has to deal with internal political unrest within the Hidden Villages. Some ninjas might be unhappy with the new leadership and try to undermine the 8th Hokage's authority, leading to a complex web of intrigue and conflict.
One idea could be Naruto going back to the time of the First Hokage to prevent a dark jutsu from being created that later causes chaos in his own time. He could use his knowledge of future events and his powerful jutsu to stop the wrongdoings.
An interesting plot for Naruto's Sandaime fanfiction might be to create a story where the Sandaime discovers a hidden power within Konoha that could either save or destroy the village. He has to make difficult decisions on how to handle this power. Maybe it's a powerful jutsu that was sealed away long ago. Additionally, you could explore his role in the creation or discovery of some of Konoha's most important landmarks or institutions. He was in charge for a long time, so he surely had a hand in many things. Or, you could write about a time when he had to make a great sacrifice for the sake of the village, like giving up his own chakra to protect a powerful seal.
One idea could be Naruto facing a new and powerful enemy that challenges the very foundation of the Hidden Leaf while he's Hokage. He has to use his unique leadership style and ninja skills to protect the village. Another could be Naruto implementing new training programs for the younger ninjas, based on his own experiences growing up.
One idea could be a story where Naruto has to face a new, powerful and mysterious enemy that has the ability to manipulate time. This would put his skills as a Hokage to the ultimate test.
A possible plot could be that there is a curse placed by an ancient enemy on the Hidden Leaf. The only way to break it is to combine the powers of Naruto and the knowledge of the Second Hokage. Naruto has to enter a spiritual realm where he can communicate with the Second Hokage's spirit. They then work together, with Naruto using his modern - day skills and the Second Hokage providing ancient wisdom, to break the curse and save the village.
Another plot could involve Naruto trying to implement new policies in the village. For example, he wants to improve the relationship between the different ninja clans. He faces resistance from some of the traditionalist clans who are afraid of change. Naruto then has to use his charisma and leadership skills to convince them.
One idea could be that she struggles with the pressure of living up to her father's legacy as Hokage. Maybe she has to deal with villains who target her specifically because of who her father is. Another could be her journey to discover her own unique ninja way that's different from her father's.