Kushina would most likely experience deep remorse. Remorse for not being there for Naruto when he needed a mother. There could also be a sense of longing in her, longing to be part of Naruto's life and to make up for all the lost time. And of course, she might be filled with self - loathing for her past actions.
Well, in a typical scenario for this kind of fanfic, Kushina might have left Naruto due to some difficult situation or misinformation. When she discovers Naruto has become 'godlike', perhaps with extraordinary powers and a great reputation. Her regret could lead to her attempting to reconnect with Naruto. She may face obstacles like Naruto's initial resentment or the complex political and social situations in the Naruto world that prevent a smooth reunion.
The main characters would be Kushina and Naruto. Kushina, as the one who regrets her past action of abandoning Naruto. Naruto, who has become 'godlike' in the fanfic, is the center of Kushina's regret. There might also be other supporting characters from the Naruto universe like Jiraiya or Kakashi who could be involved in the story, perhaps as mentors to Naruto or as those who witness Kushina's attempts to make amends.
Maybe it's because she sees Naruto's strength and how he has overcome so many difficulties despite her absence. She realizes that if she had been there, she could have made his journey easier. Also, she might regret not being able to watch Naruto grow up, miss his first steps, first words, and all those precious moments that a mother should be a part of.
Well, it could lead to her trying to make amends. Maybe she starts to secretly protect Naruto from the shadows, which adds a layer of mystery to the story. Her regret might also cause her to interfere in Naruto's relationships, like with his friends or mentors, in an attempt to make up for her past actions.
Her regret can be a catalyst for character development, not just for herself but also for Naruto. Naruto might struggle with forgiving her at first, which can be a major part of his growth. And Kushina, in her attempt to atone for her past, may interact with other characters in the village in new ways. This can lead to new relationships and plot twists, such as her influencing the village elders' opinions about Naruto or even changing some of the village's traditions related to the treatment of orphans.
Maybe she was forced to for some dark reason. Perhaps there was a threat to Naruto's life if she stayed, and she thought leaving was the only way to protect him.
If Kushina and Minato abandoned Naruto in fanfiction, it would completely change the Naruto's character development. Naruto grew up lonely and seeking recognition because he was an orphan. Without their abandonment, he might have had a very different personality, perhaps more confident from the start. Also, the relationships in the village would be different. For example, Iruka might not have had the same special bond with Naruto as he did in the original story.
One possible reason could be that she later realizes Naruto's true nature and potential. Maybe she sees how strong and kind - hearted he is despite the hardships he faced. She might regret not being there for him during his growing - up years when he needed a mother figure.
One reason could be the motherly love. Kushina always loved Naruto deep down. Leaving him means missing out on his growth and important moments in his life. Another reason might be realizing the hardships Naruto faced without her presence. She probably feels guilty for not being there to protect him from the loneliness and the many challenges he had to endure in the village.