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.
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.
Another aspect could be that the fanfiction delves into how Kushina's presence affects Naruto's relationships with others. For instance, his relationship with Sasuke could be different. With a strong maternal figure in his life, Naruto might be more stable emotionally and be able to approach Sasuke in a more understanding way, which could potentially change the whole dynamic of their rivalry and friendship.
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 might be really confused at first. He's used to people being friendly or having normal crushes, but a yandere is a whole different level. He'd probably try to talk to her and understand why she's acting so strangely.
In a fanfiction, Naruto could be training in a secluded area and Yandere Koneko stumbles upon him. She immediately falls for his spunk and the fact that he doesn't give up easily. Yandere Koneko then tries to insert herself into Naruto's life by bringing him food and small gifts. Naruto is initially confused but then realizes she has a rather extreme way of showing affection and has to gently set boundaries.
One idea could be that Naruto somehow gets trapped in a dimension where Kaguya has become a yandere for him. She tries to keep him there all to herself, while Naruto has to figure out a way to escape and also deal with her obsessive behavior.
In some fanfictions, the authors might take creative liberties and create alternate universes where Naruto and Kushina interact in different scenarios. For example, they could be on a joint mission together, and through that, we see their personalities shine. Kushina's fiery nature and Naruto's determination would make for an interesting dynamic.
One possible fanfiction could be about Kushina finding Naruto in a situation where he's lost in a forest. Maybe she uses her unique chakra sensing abilities to track him down. It could be a heartwarming story of a mother reuniting with her son.