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.
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.
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.
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.
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.
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 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.
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 the struggle between good and evil within himself. Another is redemption, as he tries to atone for his dark actions. Power and its corrupting influence is also a frequent theme.
Growth is also a prevalent theme. Both characters grow as individuals within their marriage. Shirou might become more confident and assertive, while Saber may learn to be more open and emotional. This is often shown through various events in their married life in the fanfiction, like dealing with financial issues or making big decisions together.
One common theme is the exploration of their mutual growth. They often help each other become better versions of themselves. For example, Shirou's determination might inspire Mina to be more persistent in achieving her goals.