It could also focus on some of the other Genin like Rock Lee, Neji, and Tenten. Their unique fighting styles and stories could be explored more deeply in the fanfiction.
Well, it could be about the Genin Corps going on a special mission that was not shown in the original series. Maybe they have to protect a hidden village from a new threat.
Sakura might also be a main character. She could play a role in helping Naruto with his eternal genin status, perhaps by providing medical support or strategic advice during his missions as a genin.
It could be a rogue Genin who was once expelled from the village. This Genin, let's call him Kuro, has a grudge against the village and wants to prove that he's stronger. He sees Genis and his Genin friends as obstacles to his plans of taking over the village, so he constantly tries to undermine them and steal their techniques.
I don't know for sure as I haven't read it. But since it's about genin in Naruto, it could be characters like Naruto, Sasuke, and Sakura or other newly created genin characters.
There could be a character who has a disability in the ninja world, like being partially blind but having an amazing sense of hearing or smell. Naruto would see their potential and include them in the team. Another member could be a former criminal's child who is trying to make a name for themselves in a positive way. And there could be a super - intelligent ninja who is great at strategy but lacks in practical combat experience, and Naruto would help them gain that experience.
Kakashi could also be a main character. He could be the one who has to deal with Naruto's failure in a different way than Iruka. Maybe he has to decide if Naruto is worth another chance. Also, Sakura could be involved. She might initially be disappointed in Naruto but then realizes that there is more to Naruto's failure than meets the eye and tries to support him in her own way.
It could be someone from within the village who doesn't want Naruto to succeed. Maybe a jealous rival among the other genin candidates. This person might have set up some sort of trap for Naruto during the test, causing him to fail.
Obviously Naruto would be one of the main characters. And from the Nova Corps, likely some of their leaders or important members like Rhomann Dey.
The parents or family members of the genin reserves can be significant characters too. Their family's history, values, and expectations can greatly impact the genin reserves' development. For example, if a family has a long - standing feud with another ninja family, it can add a layer of complexity to the story.
Sasuke is also a favorite. His cool demeanor and powerful abilities make him a central figure in many genin exam fanfictions. Writers often focus on his rivalry with Naruto during the exam.