Obviously, Harry would be one of the main characters as the title implies he is the first Hokage. There might also be characters from the Naruto world like some of the original ninja clans' leaders who would interact with Harry. Maybe Naruto's ancestors could be in the story as well.
The First Hokage is obviously the main character. But we may also see other important figures like his contemporaries, who were also involved in the establishment of the Hidden Leaf Village.
Yes, in some fanfictions, Naruto is depicted as the First Hokage's reincarnation. This often adds an interesting twist to the story, as it can bring in new relationships and powers related to the First Hokage's legacy.
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.
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.
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.
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 would probably first assess the damage to the village. He would send out teams to check on the injured and start the process of getting the necessary supplies for the repairs. Then, he would call a meeting with the village council to discuss security measures to prevent future attacks like Pain's. He'd want to make sure that the village is well - defended and that they have a proper warning system in place.
I'm not sure specifically as there could be many interpretations. It might be a story where the character Harry from the Harry Potter universe is somehow related to being the first Hokage from Naruto. Maybe it's a crossover fic that combines the magic of Harry Potter with the ninja world of Naruto in a really unique way.
His friends like Sakura and Shikamaru would surely support him. Sakura, with her medical skills, could be an important part of his regime, providing healthcare for the ninjas. Shikamaru, being intelligent, could offer strategic advice. Also, the younger generation of ninjas who have been inspired by Naruto would be his strong supporters, as they believe in his leadership and the new direction he could take the village in.