You can try fanfiction websites like Archive of Our Own (AO3). It has a vast collection of fanfics from different fandoms, and there's a good chance you'll find some great 'Kushina x Raikage' ones there. Just use the search function and type in the relevant tags.
One popular 'Kushina x Raikage' fanfic might be 'The Unlikely Bond'. It explores how Kushina and the Raikage, despite their different villages, form a deep connection. It could involve them meeting during a diplomatic mission and gradually realizing they have more in common than they thought.
I really like 'Kushina's Second Chance'. In this fanfic, Kushina gets a chance to come back to life and be with Naruto. The author does an amazing job of exploring their emotional bond. It has some really heart - warming scenes where they interact and get to know each other better. It also adds some interesting twists to the Naruto universe by having Kushina around.
One possible plot could be that Kushina Uzumaki, with her strong will and unique abilities from the Naruto universe, somehow crosses paths with Apollo from Greek mythology. Apollo, being the god of many things like music, prophecy, and the sun, could be intrigued by Kushina's fiery spirit. He might offer to help her with some power she has been struggling to control, and in the process, they develop a deep bond.
Maybe a story where they both find themselves in a dangerous situation outside of their respective villages. They have to rely on each other's skills and powers to survive, and in the process, a deep friendship or more could develop. For example, they could be trapped in a mysterious and dangerous forest filled with powerful beasts.
Another reason is the emotional depth. Kushina was a powerful and loving character. Writing fanfics about her and Naruto allows for a lot of emotional storytelling. There can be so many different ways to show their love for each other, from her protecting him in new ways to her teaching him things that he missed out on. It's a way for fans to explore different emotional landscapes within the Naruto world.
One possible plot could be that the Raikage and Kushina have a secret relationship. Their pregnancy could be a shock to the ninja world as it crosses village boundaries. It might lead to political tensions, with some seeing it as a way to unite the villages through their child, while others fear it as a threat to the balance of power.
They could go on a mission together in the fan fiction. During this mission, they face challenges that force them to rely on each other and understand each other's strengths and weaknesses. For example, Kushina might be strong in sealing jutsu and Naruto in his Rasengan techniques, and they combine their skills to defeat the enemy.
One of the main themes is love. Not just the mother - son love between Naruto and Kushina, but also how Kushina's love for Naruto can be a driving force in the story. For instance, in some fanfics, Kushina's love gives Naruto the strength to overcome great challenges. Also, the theme of sacrifice is common. Kushina sacrificed herself for Naruto, and fanfics often expand on this idea, showing how Naruto honors her sacrifice and what it means for his character development. In addition, there's often a theme of discovery, where Naruto discovers more about his mother and his own identity as an Uzumaki.