A plotline might involve Kushina being a 'psycho' due to some past trauma. This could lead to her having a hidden agenda within the Hidden Leaf Village. She could be secretly plotting against some of the village leaders while pretending to be a normal citizen. And Naruto, being her son, gets caught in the middle of her schemes and has to figure out how to deal with her and protect the village at the same time. It could also show how Naruto's friends react to this new, darker side of Kushina.
I'm not sure if it's a particularly popular genre. There are so many different types of Naruto fanfics out there, and this one seems rather specific. It might not be as widespread as some of the more common themes like Naruto's journey to become Hokage or his relationships with Sasuke and Sakura.
A plotline might involve Naruto traveling back in time and meeting Kushina when she was young. They could start as friends and gradually fall in love. Or perhaps there's a prophecy that links them together, and they have to fulfill it, which leads to their mating. There could also be a situation where a powerful enemy forces them to work closely together, and that closeness turns into a deeper relationship.
A possible plotline is that Naruto gets adopted by a group outside the village. He grows up learning their ways and becomes a sort of outcast from Konoha. Then, he could return to the village with a different set of skills and a new perspective. Or, he might develop a vendetta against Konoha for allowing his abandonment and try to take revenge in some way.
One possible plotline could be that Kushina is somehow mind - controlled or influenced by an evil force to harm Naruto. Maybe she's under a powerful genjutsu that makes her see Naruto as a threat to the village instead of her son.
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 He could become more withdrawn and have to fight even harder to prove himself.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.
One possible plotline could be Kushina's daily life taking care of baby Naruto. She might be shown singing lullabies to him, or just watching him sleep with a loving look. Another could be about Naruto showing early signs of his ninja abilities as a baby, like accidentally using chakra in a cute way while Kushina tries to figure out what just happened.
One possible plotline could be that Naruto is somehow transported back in time and has to fight Kushina in a misunderstanding. Maybe he's mistaken for an enemy due to some time - warp magic or jutsu gone wrong. Another could be a scenario where Kushina is controlled by an evil force, and Naruto has to fight her to break the control. Or perhaps it's a sparring match in a dream world where Naruto and Kushina test each other's skills without real harm.
One possible plotline could be that Kushina's survival changes Naruto's upbringing completely. Instead of being an orphan, he grows up with a loving mother. This could lead to him being more confident and less of an outcast in the village at a young age. He might also learn about the Uzumaki clan's secrets earlier from his mother, which could make him a more formidable ninja even faster.
One possible plotline could be that Naruto discovers some long - lost secret about his mother Kushina and decides to leave the village with her to explore this mystery further. Maybe they are on a quest to find some hidden power or knowledge that is related to Kushina's Uzumaki heritage.
In some really complex fanfic plots, there could be a prophecy or some sort of ancient power play involved. Suppose there is a prophecy that Naruto will bring great destruction in the future, and Kushina is made aware of it through some dark magic or secret organization. She might think that betraying Naruto now is the only way to save the village and the world, even though it goes against her motherly instincts. This would be a very convoluted and interesting take on the idea of her betrayal.