Maybe he realizes the exam is too dangerous or he discovers some unfairness in it. In the original Naruto story, Naruto has faced many difficulties and moral dilemmas during the Chunin Exam. So in the fanfiction, similar situations might push him to quit.
He might face disappointment from his friends and mentors. His friends who were also participating in the exam might feel let down, and his mentors, like Kakashi, could be concerned about his decision and how it will affect his future as a ninja.
There might be fanfictions that Chunin imports which are centered around relationships in the 'Naruto' world. This could be friendships that were not fully explored in the original, like the relationship between different clans' members. Or perhaps romantic relationships that add a new dimension to the story.
Growth is a common theme. Naruto and other characters are often shown developing new skills and maturing during their chunin phase. For example, in many fanfics, Naruto becomes more strategic in combat.
Friendship also remains a key theme. Even as a chunin, Naruto's relationships with his friends like Sasuke and Sakura are important. Some fanfictions might focus on how their friendships evolve in this new phase of Naruto's life. For example, they may face new challenges together and their bonds are either strengthened or tested.
Well, one thing that could happen is that Naruto, with his knowledge of future events, could easily ace the written part of the Chunin Exams. He might also be able to prevent some of the chaos that Orochimaru caused during the exams. For example, he could warn the proctors about Orochimaru's infiltration in advance.
Well, it could be about Naruto during the Chunin Exams but with a darker twist. Maybe it focuses on Naruto having to face more sinister opponents or dealing with some kind of internal struggle that's much darker than in the original story during the exams.
It could be that in this fanfic, the chunin tasks are not just about combat but also about solving complex mysteries in the ninja world. Naruto, being Naruto, would use his unorthodox methods to get to the bottom of things, which would be quite engaging to read.