Kushina might have a mixed reaction. On one hand, she might be entertained by the creativity of the fanfiction. However, if she comes across fanfictions that go against the core values of the Naruto world she knew, she would be disappointed. For instance, if the fanfiction shows Naruto using dark magic or something completely against his character, she would be taken aback. But if there are fanfictions that show Naruto in new heroic lights, she could be proud and interested.
She could be angry if the fanfiction distorts Naruto's character or the story in a negative way. For example, if Naruto is portrayed as a bad person in the fanfiction, Kushina, as his mother, would not like it at all. But if she finds some positive and creative fanfictions, she might be impressed by the fans' imagination.
She could also be amused in some cases. If the fanfiction has some really out - there but harmless ideas about Naruto, like him having a strange hobby that doesn't really change his fundamental good nature, Kushina might find it funny and just shake her head at the creativity of the fanfiction writer.
Kushina returning to find crazy Naruto fanfiction could lead to a whole range of reactions. Firstly, she might be appalled at how some fans have twisted the story of her son. On the other hand, she could be amused by the wild ideas. If she finds fanfictions that involve her own backstory in strange ways, she might feel a bit violated but also intrigued. For instance, if there's a fanfiction where her relationship with Minato is completely different from what it really was, she'll have a strong reaction. But overall, it would be a very interesting encounter between her and the world of Naruto fanfiction.
Kushina would be extremely worried. She would rush to Naruto's side immediately and start using her medical knowledge or any available resources to help him get better.
Naruto might be extremely heartbroken. He has always looked up to his mother, and being disowned by her would be a huge blow. He could become withdrawn and start to question his own worth.
In this fanfiction, the plot might involve Kushina coming back to help Naruto in his toughest battles. Maybe she uses her unique chakra abilities that Naruto has inherited to give him an edge. She could also provide emotional support, as Naruto has always longed for his mother.
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.
Naruto would probably be really confused at first. He's used to a loving mother figure, not a yandere one. He might try to talk to her and understand why she's acting so strangely.
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.
Minato is often paired with Kushina in these fanfictions. Their relationship was a significant part of the Naruto story, so many fanfictions expand on their love story. Another popular pairing is Kushina and Naruto. It could be a story where Kushina is brought back to life and gets to be a part of Naruto's life in a more direct way.
One possible plotline could be Kushina's reunion with Naruto and how she uses her skills and wisdom to help him in his battles. For example, she might teach him some new jutsu that only she knew. Another plotline could involve Kushina exploring the new ninja world that has changed so much since her time. She could interact with characters like Sasuke and Sakura, and have an impact on their development as well.