One common theme is the idea of redemption. Maybe in some fanfictions, Minato or Kushina have something they feel they need to make up for in the afterlife, and it involves helping Naruto in some way. Another is the theme of growth. Naruto grows up without his parents, but in fanfiction, they can be a part of his growth process in some supernatural way. And of course, the theme of heroism runs through a lot of these stories, as Minato was a hero, and Naruto becomes one too, and Kushina was a strong kunoichi.
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.
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 possible fanfiction could be about Minato and Kushina watching over Naruto from the afterlife as he grows into a great ninja. They could be his silent guardians, influencing his decisions in subtle ways.
One common theme could be rewriting the Naruto story with different character development for Minato and Kushina. Maybe they are portrayed as more flawed or making different choices that lead to alternative storylines.
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 the idea of sacrifice. In many fanfictions, either Minato or Kushina is willing to sacrifice themselves for the other. It shows the depth of their love. Another theme is the power of their bond. Their relationship is often depicted as something that gives them strength, whether it's in battle or in dealing with the politics of the Hidden Leaf Village.
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 family dynamic in fanfiction could be quite interesting. Minato could be a strict but fair father, pushing Naruto to be the best ninja he can be. Kushina would balance this with her motherly love, always there to comfort Naruto when he fails. They would celebrate Uzumaki traditions together, and Naruto would learn about his parents' pasts. He would look up to Minato as a great Hokage and to Kushina as a strong - willed Uzumaki. Their family would be a source of strength for Naruto as he grows up in the ninja world.
A great fanfiction concept could be that Minato and Kushina are resurrected during a crucial battle in Naruto's life. Their presence gives Naruto a huge morale boost. They fight side by side with Naruto against a powerful foe. After the battle, they decide to stay in the village and help Naruto rebuild and strengthen it. This would not only be a great action - packed story but also a heartwarming family reunion story.
On a more personal level, they could face internal guilt. As Naruto grows up and achieves great things on his own despite their neglect, Minato and Kushina may realize what they've missed out on. This realization could cause them a great deal of internal turmoil and regret for not being there for their son when he needed them.