One common theme is forbidden love. Since a goddess and a human are from different realms, their relationship is often seen as going against the natural order. Another theme is power imbalance. The goddess usually has great power, and the human may be weak in comparison, which creates an interesting dynamic.
First, establish the world where the goddess and human interact. Maybe it's a modern world where the goddess is disguised as an ordinary person. Then, create a conflict. For example, the goddess's divine duties conflict with her love for the human. Also, add some magical elements, like the goddess using her powers to protect the human from danger. Build their relationship gradually, from initial attraction to deep love.
One common theme is forbidden love. Since they are from different realms, their relationship is often not allowed. Another is sacrifice. The goddess or the human may have to sacrifice something important, like the goddess giving up her immortality for the human. Also, discovery is a theme. The human discovers the world of the divine through the goddess, and the goddess discovers the beauty of human emotions.
One popular 'goddess x human fanfiction' could be 'A Goddess's Love for a Mortal'. It tells the story of a goddess who falls in love with a simple human. Their love defies the rules of the divine world. Another is 'The Goddess and Her Human Protege', where the goddess takes a human under her wing and an unexpected bond forms between them.
The goddess character herself is also very popular. Depending on how the author writes her, she could be a powerful and wise being, like Tsunade but with divine powers. Fans are often interested in her backstory, her powers, and how she views Naruto.
Definitely not. Ethical considerations play a huge role here. Dolphins are intelligent and sentient creatures, but they are not on the same level as humans in terms of the kind of relationships we can have. Fanfiction that blurs these lines can be disturbing and goes against the values of respecting animals as they are, without creating inappropriate relationships in the fictional space.
Perhaps the goddess has the ability to see different timelines. She sees a future where Goku is the most important figure in the universe's survival. So she travels back in time to meet him when he's still young. She becomes his mentor, guiding him not just in power but also in understanding the greater cosmos. As Goku grows up, he starts to see her not just as a mentor but as a love interest, and they develop a relationship that spans different stages of Goku's life.
A plot might involve Naruto being in a tough battle and on the verge of defeat. Goddess Inari appears and saves him. Out of gratitude, Naruto offers to help her with a task in the spirit world. As they work together, they start to respect and care for each other more deeply, and Naruto learns new spiritual techniques from her.
Ichigo can be developed by giving him new powers or abilities related to his connection with the goddess. This could be a way to show his evolution in the fanfiction. The goddess, on the other hand, can be developed by exploring her backstory. Why is she in the human world? What are her goals? By answering these questions, her character becomes more complex and interesting.
One idea could be that Ichigo discovers he has a hidden connection to the goddess realm. Maybe he is a chosen one to protect the goddess from an ancient evil. For example, he could have special powers that are awakened only in the presence of the goddess.