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.
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.
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.
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.
One interesting plot twist could be Shirou having a different Servant. Instead of the usual ones, he might summon a Servant from a completely different era or mythology, which changes the dynamics of the Grail War. For example, a Servant from an African mythology who has unique abilities that no one in the traditional Grail War has seen before.
One interesting plot idea could be that Artoria Archer and Shirou Emiya are forced to team up in a strange magical tournament. They start off distrusting each other, but as they face various powerful opponents, they gradually learn to work together and understand each other's motives better. For example, they might encounter a foe who can manipulate time, and they have to rely on Artoria's combat experience as an Archer and Shirou's determination to find a way to defeat the enemy.
One plot idea could be Dark Shirou in a post - apocalyptic world where he uses his skills to survive and rule a small territory. Another could be his journey to find a way to reverse his dark transformation. Or a plot where he has to face his darker self in a psychological battle within his own mind.
One plot idea could be Shirou Emiya finding himself in the world of Fairy Tail and having to adapt to the magic - based society. He might try to use his Reality Marble in unique ways to combat the dark guilds. Another could be him teaming up with Natsu and his friends to take on a powerful, otherworldly threat that has somehow entered the Fairy Tail universe. He could also learn new forms of magic from the wizards in Fairy Tail, like learning how to incorporate Fairy Tail - style elemental magic into his existing abilities.
One idea could be that Archer is transported to an alternate universe where Shirou has a completely different personality. Maybe Shirou is a villain in this world, and Archer has to figure out how to deal with him. Another plot could involve them teaming up to fight a powerful, new enemy that neither could defeat alone. For example, a powerful mage from a different dimension who wants to conquer their world.
Another plot could involve Shirou reincarnated in a modern superhero world. He might struggle to fit in at first, but then realizes his abilities can be used to fight super - villains. His noble phantasm could be adapted in a unique way to combat the new threats in this world.
Another plot could be that Shirou discovers a new, highly competitive gamer community. He starts from the bottom but his determination and strategic thinking, which he has from his heroic deeds, help him rise through the ranks. He might make new friends and rivals in this community.