A common theme is over - protectiveness. Since Kushina is yandere, she often tries to shield Naruto from any harm or perceived threats. Another theme is possessiveness, where she might not like Naruto getting too close to other characters.
I'm not sure if there are extremely famous 'yandere Kushina x Naruto fanfic' but when looking for them, also consider looking at fan communities on Reddit dedicated to Naruto fanfiction. People there might be able to point you to some interesting stories. Maybe there are some hidden gems that haven't gained wide popularity yet but are really good in terms of exploring the yandere Kushina and Naruto relationship.
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.
Maybe the fanfiction could start with Kushina being resurrected in some way and immediately developing this yandere love for Naruto. Naruto, being the kind - hearted person he is, tries to understand her but is constantly baffled by her extreme behavior. For example, she gets jealous when Naruto spends time with his sensei, Kakashi, and tries to sabotage their training sessions.
For Naruto, you could show his growth in terms of his leadership skills when he's with Naruko and Kushina. He could learn to be more patient and understanding, especially when Naruko might have a different approach to things. For Naruko, her character could be developed by showing her struggle to be recognized in a world where Naruto already has a reputation. Kushina could be developed by exploring her motherly side more deeply, as she interacts with both Naruto and Naruko.
Minato might be a common character. Since he is Kushina's husband in the original story, his presence can add an interesting dynamic. Also, Jiraiya could be there. He was Naruto's mentor and his interactions with the harem situation could be quite humorous.
In such a fanfiction, the relationship between Naruto and Kushina would be one of mother - son love and mutual respect. Kushina would be protective of Naruto and proud of the man he has become. Naruto would look up to Kushina and value her wisdom. With Hinata, Naruto's relationship would be that of a loving couple. Hinata would support Naruto in all his endeavors. When it comes to Kushina and Hinata, Kushina would likely see Hinata as a worthy partner for Naruto and Hinata would respect Kushina for being Naruto's mother and a strong kunoichi.
One possible reason could be a misunderstanding. Maybe Minato thought Kushina had done something to endanger Naruto when in fact she hadn't. In the heat of the moment, he lost his temper and slapped her.
One common theme is the extreme nature of the yandere's love. It's often possessive and can lead to some intense and sometimes dangerous situations for Naruto. Another theme is the struggle between Naruto's own values and the yandere's demands. For example, Naruto might value friendship and freedom, while the yandere wants him all to themselves.
One common theme is obsession. The yandere character is often overly obsessed with Naruto. For example, they might follow him everywhere or try to keep him away from other people. Another theme is possessiveness. They see Naruto as their own and get jealous easily.