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.
Another possible main character could be Iruka. He has always been important in Naruto's life. In this fanfiction, he might play an even bigger role in helping Naruto deal with the emotional pain of being left by his mother. He could be like a surrogate family member for Naruto, providing the love and support that Naruto lacks because of Kushina's absence.
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.
It could be someone who took advantage of Kushina's leaving. Maybe a rogue ninja who wants to use Naruto's vulnerability to their advantage. They could try to turn Naruto against the village by making him believe that the village drove Kushina away.
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 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.
One possible plotline could be Naruto's struggle with abandonment issues. Since Kushina left, Naruto might have a harder time finding his place in the village. He could become more withdrawn and have to fight even harder to prove himself. Another could be the search for Kushina. Naruto might embark on a journey to find out why she left and try to bring her back.
Jiraiya could also be one of the main characters. He was Naruto's mentor and had a connection with Kushina's family. He could play a role in helping them adjust to the time travel and also use their knowledge to his advantage in some ways, like getting intel on future threats.
Obviously, Kushina is one of the main characters. Naruto is also a central figure as her son. Since it's related to the Naruto universe, characters like Sasuke, Sakura, Kakashi might also be important in the fanfiction. They could interact with Kushina in various ways, like Sasuke might be skeptical of her at first, or Sakura could be eager to learn from her.
Kushina is a central character as the whole concept is about her remembering things related to Naruto. Naruto, being the subject of her memories, is also a main character. There might also be other characters like Minato, Naruto's father. Since Kushina was married to Minato, he could be part of her memories too. Also, Naruto's friends like Sasuke and Sakura could potentially be in the fanfiction as they are important people in Naruto's life and thus might be part of Kushina's recollections.
I'm not sure specifically as I haven't read the 'left behind the novel'. But typically, in a novel with a title like that, there might be a character who is the one left behind. It could be a child, an adult, or even an animal in some cases.
Obviously, Natsu would be a main character since he's the one left behind. There could also be other characters from the original series who are relevant to the story, like his friends who left him, such as Lucy, Gray, and Erza. Maybe some local characters in Tenrou could also play important roles in the fanfiction.