Perhaps there's no traditional 'antagonist' in the sense. Instead, Naruto's own self - doubt could be the main 'antagonist' in this fanfiction. His insecurities and past traumas might resurface during the test, causing him to second - guess himself at every turn and ultimately leading to his failure.
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.
Perhaps in this fanfiction, Naruto fails the genin test due to external factors. Say, there's some sort of interference from a hidden enemy. This enemy might have used a genjutsu on Naruto right before the test, making him see things that aren't there or confusing his mind. So, even though Naruto has the skills, he just can't perform well during the test and fails as a result.
In such a fanfic, perhaps Naruto is overly nervous during the test. Maybe he fails the written part because he didn't study enough about basic ninja knowledge. For example, he could mix up different types of jutsu classifications. Another possibility is that he fails the practical part. He might lose control of his chakra while performing a simple jutsu, like the Clone Jutsu, which is crucial for genin. This could lead to him not creating enough clones or creating unstable ones that disappear quickly.
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.
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.
Naruto's character could develop in a more humble way. After failing the test, he'd see that he has a long way to go. He might start to observe his classmates more closely, like how Sakura studies hard or how Sasuke is so focused during training. This could make him less impulsive and more strategic in his approach to becoming a ninja. He'd also learn the importance of preparation and might even seek help from unexpected sources, like some of the older ninjas in the village who have different techniques to teach.
Well, perhaps in this fanfiction, Naruto fails the bell test because of external distractions. For example, some unexpected enemy ninjas could show up during the test. Naruto, being the hot - headed and impulsive one, gets distracted by the need to protect his teammates from these new threats instead of focusing on the bells. Also, it could be that Kakashi is much stricter in this version. He doesn't give any leeway or second chances, and Naruto just doesn't meet the mark.
Well, one possible plot could be that Naruto fails the test due to his overconfidence. He might rush through the practical parts without properly showing his skills. For example, in the clone jutsu part, he could create clones that are too obvious and lack the finesse required. This would then lead to him having to face the disappointment of his peers and his own self - doubt, which could be a major part of the fanfic as he tries to overcome it and prove himself.
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.
Maybe it's a corrupted god. Let's say Hades in this case. He's always been a bit of an outcast among the gods. He decides to use Percy as a test subject in a plot to gain more power in the underworld. He subjects Percy to all sorts of trials and tests that are meant to break him down and extract his power. Percy has to fight against Hades' dark magic and find a way to escape his clutches.
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.