Maybe Naruto is left behind in training because the instructor misjudges his abilities. As a result, Naruto starts training with a retired ninja who teaches him some old - fashioned but effective techniques. This gives him an edge when he finally rejoins his group.
It could be that Naruto is left behind as a punishment for some prank he pulled. While left behind, he meets a strange animal that has some connection to the Sage of Six Paths. Through his interaction with this animal, Naruto gains new insights into chakra and how to control it better.
It could be a jealous fellow trainee. This person might spread false rumors about Naruto, which leads to him being left behind in training. The jealous trainee doesn't want Naruto to get stronger and be more popular.
Well, it might be about Naruto growing up without Kushina's presence. Maybe he has to face challenges alone and discovers things about his mother in the process.
Perhaps in this fanfiction, Naruto's family leaves him because they fear his godlike abilities. Naruto then has to prove that he can use his powers for good. He could start by helping the villagers who once shunned him. This would lead to a series of events where he gradually gains the respect and understanding of those around him, despite his family's absence.
Perhaps there was a dark secret about Naruto's origin that was revealed in a way that made others fear him. For example, if it was discovered that he had some connection to a feared enemy or an ancient evil. This could lead to his betrayal as people's fear overrode their previous loyalty to him. Also, jealousy among his peers could be a factor. If Naruto was constantly getting stronger and more popular, some might have become envious and were easily convinced to turn against him.
One possible plot could be that Naruto feels abandoned by Sakura after she chooses Sasuke over him. Naruto then goes on a journey of self - discovery, training harder than ever to prove his worth not just to Sakura but to the whole village. He might face new enemies alone and form unexpected alliances, all while dealing with the emotional pain of being left behind.
As he grows up, Naruto could become very angry. He might develop a sense of abandonment and this could fuel his determination to become stronger. He would want to prove himself to his mother, wherever she is. In this fanfiction, his anger could be a driving force in his character development.
Well, it could be about Natsu being accidentally or deliberately left in a certain place. Maybe his friends go on a mission without him, and he has to find his own way back or prove himself while being left behind. It might also involve his feelings of abandonment and how he overcomes them.
One possible plot could be that Anko takes on a young and inexperienced ninja as her apprentice. She could start with basic physical training, like running through the forest while dodging various traps she sets up. Then, she might move on to teaching ninja techniques, especially those related to her own special skills in snake - based jutsu.
One possible reason could be that Kushina was forced to leave Naruto due to some external threat she thought would endanger him more if she stayed. For example, if there were powerful enemies after her and she feared they would use Naruto to get to her.
I think Minato could also be a main character. As Naruto's father, he might play a role in Naruto's life even though Kushina is absent. He could be a guiding figure or a source of mystery for Naruto, especially if Naruto discovers things about his father in relation to his mother's absence.