A prevalent theme is the battle between his human morality and vampiric instincts. His previous moral code as a human might clash with the new, more primal urges of a vampire. This internal conflict can drive the plot forward in many vampire shirou fanfictions. Also, the idea of redemption or acceptance of his new state is also a common theme that runs through a lot of these stories.
Love is a very common theme. Their deep and often complex relationship is explored in many ways, from the budding of new love to the strengthening of an already existing bond.
One common theme is self - discovery. As Shirou delves into swordsmithing, he often discovers new things about himself. Another is the pursuit of perfection. Since swordsmithing requires precision, Shirou is often on a journey to create the perfect sword. And there's also the theme of legacy, as his swords can leave a lasting mark on the world or on those who wield them.
One common theme is the relationship between them, whether it's a budding romance or a deep friendship. Another theme is their participation in the Holy Grail War, with how they strategize and support each other during battles. Also, their different personalities - Shirou's selfless nature and Tohsaka's more calculating side - often lead to themes of conflict and resolution as they interact and learn from each other.
One common theme is love. Their relationship is often explored in different ways, from a slow - burn romance to an instant connection. Another theme is loyalty. They are both loyal characters, and in fanfiction, this loyalty is often tested and reaffirmed.
Another theme is the adaptation of her character traits. A female Shirou may still hold onto the original Shirou's sense of justice, but it could be expressed in different ways. Maybe she is more empathetic as a female, which changes how she approaches helping others in the fanfiction.
One common theme is power. Shirou often gains new powers or abilities through his connection with the dragon. Another is adventure, as they usually go on quests together.
Love is a very common theme. Their relationship, whether it's a budding romance or a deep, abiding love, is often explored. Another theme is heroism. Since both are heroic characters in their own right, their heroic deeds and how they support each other in that aspect is often written about.
The theme of changing fate also frequently appears. Shirou going back in time means he has the opportunity to change the fate of his friends, his city, or even the whole world. He has to make decisions that could completely rewrite the future, and this struggle between his desire to change things and the potential consequences is often explored in these fanfictions.
Another theme is identity. As Shirou enters the Miyuverse, he often questions who he is in this new context. He may compare his values from his own world to those in the Miyuverse. In a particular fanfiction, he struggles with whether his heroic ideals still hold true in this strange new place.
One common theme is the clash of two different worlds. Shirou comes from the Fate universe with its own set of rules, magic, and values, while the Sekirei world has its unique system of Sekirei, Ashikabi, and battles. Another theme is adaptation. Shirou has to adapt to the new rules and environment of the Sekirei world. For example, understanding how to form bonds with Sekirei as an Ashikabi.