Another theme could be family. Since they are the parents of Naruto, fanfics might explore how they would have raised Naruto together. This includes their parenting styles and the values they would have instilled in him.
Protection is also a theme. Minato was known for protecting the village, but in the context of their relationship, he was always there to protect Kushina. Fanfics may expand on this aspect, showing different scenarios where Minato has to protect Kushina from various threats.
The theme of heroism also appears. Minato was a great Hokage and Kushina was a strong kunoichi. Fanfics may depict them facing various threats together, showing their heroic sides and how they supported each other in difficult situations.
Well, loyalty is a big theme. In these fanfics, Minato and Kushina are shown to be extremely loyal to each other. No matter what difficulties they face, they stay by each other's side. Another theme is the exploration of their different backgrounds and how they complement each other. Minato's calmness and Kushina's passion make for an interesting dynamic in their relationship.
One possible aspect to analyze in a Kushina x Minato fanfic could be their relationship dynamics. Their love was strong and Minato was always very protective of Kushina. Fanfics might explore their courtship in more detail, like how they first fell for each other.
Love is a very common theme. Their relationship, from falling in love to building a family, is often explored. For example, in many fanfictions, their first meeting and the gradual development of their feelings are depicted in great detail.
One common theme is sacrifice. Since they are ninjas, they often have to sacrifice their personal time together for the sake of the village. For example, Minato may have to go on dangerous missions, leaving Kushina worried but still supporting him.
The theme of trust also pops up often. Since Minato entrusted Kakashi with important tasks, fanfics often explore the depth of that trust. And there's also the theme of loss. Minato's death had a huge impact on Kakashi, and many fanfics deal with how Kakashi copes with that loss while still holding onto the memories and lessons from Minato.
One common theme is loyalty. Akitsu is often portrayed as being extremely loyal to Minato, and this loyalty is a key aspect of their relationship in the fanfics. For example, in many stories, she will go to great lengths to protect him.
Time - travel is also a frequent theme. For example, Minato might be brought to the present or Naruto sent back in time to meet his father. This allows for a lot of interesting interactions and character development. And of course, the theme of mentorship is there. Minato was a great shinobi, and fanfics often show him mentoring Naruto and passing on his skills and wisdom.
The theme of legacy is also quite common. Since Minato is the Fourth Hokage, fanfics might explore how he wants Naruto to carry on his legacy. And Kushina's strong will also gets passed on to Naruto in these stories. They also often show the simple joys of family life, like spending time together at home, which is a contrast to the dangerous ninja world outside.
One common theme is isolation. Naruto would be all alone without his parents, and this would shape his relationships. Another theme is revenge. In some fanfictions, he might seek revenge on the village for his abandonment.