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.
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.
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.
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.
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.
In another scenario, the fanfiction could be set in a post - apocalyptic world. Shirou and the female archer are part of different survivor groups. They meet during a scouting mission. At first, there is tension between them due to the rivalry between their groups. However, as they realize that they share a common love for archery and a desire to rebuild the world, they decide to work together to bring peace and restore the art of archery in the new world.
One possible plotline could be Shirou finding himself in Westeros and using his skills as a swordsman to get involved in the power struggles. Maybe he becomes a knight in someone's service.
In such a fanfiction, Shirou - turned - Avenger could be on a path of vengeance against those he believes have wronged him or his loved ones in the past. This could lead to a series of intense battles where he uses his new - found powers in a more brutal way compared to when he was just Shirou. And his relationships with other characters would also change drastically, like with Rin or Saber, who might be initially shocked and then try to bring him back to his former self.
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.