First, you can give Shirou a new backstory. Maybe he has a different family history or a unique experience that shaped him. This will make his character more distinct. For instance, if he was raised in a different part of the world, his views on heroism might be different.
Another plot idea might be to focus on Shirou's internal struggle during the Grail War in more depth. Show how his ideals are tested not just in battle, but also in his interactions with other Masters and Servants. Maybe he has to make some really tough moral choices that could change the course of the war.
One aspect that makes it unique is the character interactions. Imagine Shirou, with his strong sense of justice from the Fate series, interacting with the RWBY characters like Ruby, Weiss, Blake, and Yang. Their different personalities clash and blend in interesting ways. Also, the fanfic can explore how Shirou's abilities, like his projection magic, work in the RWBY world which is full of Dust - based technology and different combat styles.
Servants are also often main characters. For instance, Saber, with her noble and powerful presence. In the fanfic, she might have different relationships with other characters compared to the original. Her backstory and her connection to the Grail War are important elements that make her a main character.
They play with the differences in the universes. Let's say a crossover with a post - apocalyptic world. Shirou's skills and values would be tested in a whole new way. The writer has to create new challenges for him, like how he would deal with limited resources while still trying to uphold his heroic nature. Also, they might introduce new characters from the crossover world that can either help or hinder Shirou, adding more depth to the story.
The relationships between the characters can also be an interesting concept. In the fanfic, characters who may not have interacted much in the original could form strong alliances or rivalries. It can show how different personalities clash or complement each other in unexpected ways.
Another plot could involve Shirou getting a glimpse into the past of the Grail War through some sort of magical artifact. He discovers secrets that were long buried and this knowledge could either help him in the war or put him in greater danger as others may want to silence him.
In fanfiction, Shirou could be exposed to a different moral code during the 4th Grail War. Maybe he meets a group of mages who have a more flexible view on right and wrong. This exposure could make him question his own beliefs and lead to a significant change in his character as he tries to find his place in this new moral landscape.
The character of Shirou himself. He has a complex personality and set of ideals. In a war fanfic, his struggle to uphold his beliefs while facing the chaos and destruction of war is really engaging.
One main theme is heroism. Shirou has his own concept of being a hero, and the fanfic often explores how he tries to uphold that in the chaotic Fourth Grail War. For example, he might face difficult choices where his idea of heroism is tested.
One popular 'emiya shirou war fanfic' is 'The War of the Redemption'. It focuses on Shirou's journey during a war, his growth, and how he uses his abilities to protect those he cares about. The battles are well - described and it really delves into his character.