One popular theme is the exploration of their relationship in alternate universes. For example, in some fanfictions, they might be in a modern setting rather than the original Fate/stay night universe. Another theme could be the growth of their bond through various challenges. They might face new enemies together, which deepens their connection. Also, the theme of self - discovery within their relationship is common. As they interact more, they learn more about themselves and what they truly want from each other.
Well, a common theme is the power dynamics between them. Since Saber is a powerful Servant and Shirou is a human with his own set of skills. Fanfictions often play with how this power difference affects their relationship. It could be that Shirou has to prove himself to Saber in some way, or Saber has to learn to rely on Shirou despite her strength. Another theme is the idea of sacrifice. In the original story, there are elements of sacrifice, and fanfictions expand on this. Maybe Shirou is willing to sacrifice more for Saber's sake, or vice versa.
Some fanfictions focus on the cultural differences between Saber and Shirou. Saber comes from a different time and place, and this can lead to interesting scenarios. For instance, she might not understand some modern concepts that Shirou takes for granted, and they have to work through these misunderstandings. Also, the theme of redemption can be present. If either Saber or Shirou has made mistakes in the past, their relationship can be a means of finding redemption. They support each other and help each other move forward from their past wrongs.
Chapter 300, ", Battle for Command Authority", is the clearest early answer because the buildup finally cashes out in a real clash. The chapter turns that into something concrete by moving from Although it could not be said that the entire Planetary Defense Army... to Cai Minjin stood up in surprise, even forgetting to put away his gun..
Chapter 405, ", The Defector Interstellar Warrior", is the clearest early answer because the buildup finally cashes out in a real clash. The chapter gives readers a real answer by opening on On Rage Owl Star, within the Warband Monastery. and ending on Furious, he had the Planetary Defense Army commander who had ordered.... In story terms, the danger stops being abstract and the conflict around power growth becomes immediate, forcing the surrounding cast to react under pressure.
Chapter 308, ", Administration", is the clearest early answer because it turns Alliance's situation into a concrete plot event. The chapter turns that into something concrete by moving from The recovered western part of the continent was divided into three... to This is quite rare, and the contrast is telling..
Chapter 257, ", They Struck First", is the clearest early answer because it turns Gu Hang's situation into a concrete plot event. The setup is After a pause, Wang Qi continued, "Moreover, this is just a simple..., but the pressure keeps building through Here we are on Heijian Star, which has an owner, which is governed by....
Chapter 332, "The Titan is completely finished", is the clearest early answer because it turns Interstellar Warriors's situation into a concrete plot event. The setup is The Phoenix Battle Group had been a part of this battle right from..., but the pressure keeps building through As such a reliable force, Perbov of course wouldn't overlook them..
Chapter 409, ", Atonement Expedition, Independent Kingdom", is the clearest early answer because it turns Extinct Soil Star's situation into a concrete plot event. A useful checkpoint is the way the scene starts with But the probability of that should not be considered high. and follows through with The New Year period is actually very busy, but I'll try my best to....
Chapter 371, ", The Right Path", is the clearest early answer because it turns Gu's situation into a concrete plot event. The chapter turns that into something concrete by moving from Upon receiving orders to split his forces and head to two different... to Then, after conducting his own random inspections, he had to admit.... What makes it stick is that the chapter closes with Why be so earnest?, which gives family politics a stronger narrative position and pushes the power growth onward.
Chapter 350, ", Tens of Millions of Troops", is the clearest early answer because it turns Interstellar Warriors's situation into a concrete plot event. The setup is Greetings, this is the first time we meet, so allow me to introduce..., but the pressure keeps building through I am Lambert Hodgson, the Chief Justice of the Alliance..
Chapter 657, "a major turning point", is the clearest early answer because it turns Black Boxes's situation into a concrete plot event. The chapter turns that into something concrete by moving from Since the Korolya incident erupted eight months had passed, and Gu... to This also meant that the utilization rate of the entire space had to....
Chapter 262, "Settlement of the Heijian Star Campaign", is the clearest early answer because it turns Luman Kingdom's situation into a concrete plot event. It opens with The Luman Kingdom wasn't large; in terms of size, it was about as big... and closes on However, before that, at the start of the fighting, Perbov led three....