A great plotline could be that Naruto's early graduation catches the attention of some of the village's more secretive or powerful ninja. They start to keep a closer eye on him, either to protect him or to see if he can be of use to their own agendas. Maybe some old enemies of the village think he could be a weak point since he's so young and inexperienced, despite his early graduation.
One possible reason could be his natural determination. Naruto is highly motivated to become Hokage. In fanfiction, this drive might lead him to study extra hard and show extraordinary progress, convincing the academy instructors to let him graduate early. For example, he could be spending all his free time training and studying ninja techniques instead of goofing off like some of his peers.
There could be a plotline where Naruto and Sakura discover a hidden part of the academy that holds ancient ninja secrets. They have to work together to unlock the secrets, and along the way, they face various challenges and also grow closer as friends. This could also lead to them learning new powers or techniques that are unique to this hidden part of the academy.
One possible plotline could be Naruto quickly learning to control Kurama's power earlier than in the original story. This might lead to him being able to defeat stronger enemies sooner and gain more respect in the village. Another could be Kurama and Naruto forming a deeper bond right away, and Kurama sharing some of his ancient knowledge with Naruto, which helps Naruto in his ninja training.
One interesting plotline could be Kakashi teaching Naruto basic chakra control in secret. Naruto, being young and eager, quickly grasps some concepts but struggles with others. This could lead to some comical moments as Naruto tries to impress Kakashi with his progress.
One possible plotline could be that Naruto and Kurama start training together early on. This would give Naruto a huge power boost right from the start. He could learn to control Kurama's chakra in new and unique ways, leading to him being able to perform more advanced jutsu earlier in the story.
One interesting plotline could be Allison and Luther trying to reconnect after years of being apart due to the chaos in their lives. They might start by sharing memories of their childhood in the Academy that they had long forgotten. For example, they could recall a particular training session where they had to rely on each other to complete a difficult task. Another plotline could involve them facing a new threat together and having to use their unique abilities in harmony. Allison's power of rumor manipulation and Luther's super strength could combine in unexpected ways to defeat the enemy.
Perhaps his early birth gives him an edge in his ninja training. He could start learning advanced techniques sooner than in the original story. This might cause the village elders to take a different approach to him, either seeing him as a greater threat or a more valuable asset right from the start.
In some fanfictions of 'harry discovers magic early', he might accidentally perform magic in front of the Dursleys much earlier than in the original story. This could lead to either extreme fear or a strange kind of acceptance from them. Maybe they try to hide it from the wizarding world for their own gain. For example, Vernon might see it as a way to make money if he can control Harry's magic.
A third plotline could be Seth and Caleb going on a quest together outside of the academy. They might be searching for a lost artifact that has the power to change the fate of the academy or the entire magical world. Along the way, they face various challenges and obstacles that test their friendship and their magical abilities.
One interesting plotline could be the development of relationships between the characters. For example, how they form deeper bonds over time. Another could be the exploration of their individual powers and how they use them to overcome challenges. Maybe there are also plotlines about secrets within the academy that are gradually uncovered.