The main characters are clearly Shirou and Zelretch. Shirou is often known for his heroic ideals and his journey in the world of magic and heroism. Zelretch, on the other hand, is a very powerful and mysterious mage.
In the 'shirou meets zelretch fanfiction', Shirou might learn some unique magic from Zelretch. Since Zelretch is a powerful mage, he could have a great influence on Shirou's growth as a mage.
Perhaps Zelretch decides to show Shirou different timelines or alternate realities. This could be a way to both amuse himself and also give Shirou a new perspective. It could also lead to Shirou having to make some tough decisions about his own path based on what he sees in these other timelines.
If Zelretch, in his boredom, decides to meddle with Shirou's life, it could have a huge impact on the story's universe. For example, if he gives Shirou a new power, Shirou might use it to change the course of events in his world. This could lead to a ripple effect, changing the fates of many other characters as well.
One common theme could be identity. Since they are gender - switched, they have to re - evaluate who they are and how they interact with the world. Another theme might be friendship or partnership. They often team up in these fanfictions to achieve a common goal. For example, they could be on a quest to save a magical world.
A plotline could involve Zelretch having a specific reason for turning Shirou female. It could be part of a grand plan to save the world or to correct some imbalance in the magical realm. And as Shirou is now female, she may discover new magical abilities or limitations that come with her new form. This could lead to a whole new adventure as she tries to master these changes while dealing with the overarching plot.
One interesting fanfiction could be where they are female and have a unique adventure in a magical realm. Maybe they team up to fight an ancient evil. It could explore their different personalities in this new gender - switched form, like Shirou's female version being more nurturing yet still having that strong - willed determination, and Zelretch's female form being more mysterious and alluring while still having her powerful magic.
One possibility in the 'naruto meets zelretch fanfiction' is that they team up to face a common enemy. Zelretch with his powerful magic and Naruto with his ninja abilities could create a very interesting and powerful duo. They might also learn from each other, with Naruto teaching Zelretch about the values of the ninja world like loyalty and perseverance, and Zelretch showing Naruto the vastness of the magical world.
In the 'naruto meets zelretch fanfiction', neither might be truly dominant. They could be equals. Naruto has shown great growth and the ability to overcome powerful foes. Zelretch has his vast magical knowledge and power. They might end up respecting each other's strengths and working together without one being over the other. For example, they could combine their powers in unique ways, like Naruto using his shadow clone jutsu in tandem with Zelretch's magic to create a more powerful attack.
I'm not sure about specific 'zelretch turns shirou female' fanfiction stories off the top of my head. You could try searching on popular fanfiction websites like Archive of Our Own or FanFiction.net. They usually have a wide variety of such niche fanfictions.