One idea could be Emiya in an alternate timeline where he has to team up with his younger self. It would be a journey of self - discovery and mentorship. Another could be Emiya getting trapped in a dream world created by a powerful mage, and he has to find his way out while facing his inner demons.
Well, a great plot could involve Emiya being sent to a completely different dimension within the Fate Grand Order universe. There, he has to adapt to new rules and enemies. Maybe he meets a version of a character that is completely different from what he knows. For example, a Saber who is a villain in that dimension. He then has to figure out how to get back to his own world while also dealing with the chaos in this new one.
One idea could be to have the Servants from Fate Grand Order interact with the events and characters of Fate Zero in unexpected ways. For example, Mash could end up in the middle of the Fourth Holy Grail War and try to prevent some of the tragedies from happening. She might team up with a young Kiritsugu, using her knowledge from the future to guide him. Another could be that a Grand Order Servant like Artoria Alter gets involved in the war and changes the power dynamics completely, as her dark and powerful nature would be a wild card in the already complex Fate Zero setting.
The theme of redemption can also be found. Given Shirou's past mistakes and his journey to become a better person, fanwriters like to delve into this aspect. They might create scenarios where he has to face the consequences of his actions from his past life and find a way to atone. Some stories also explore his relationship with the different factions in the Fate Grand Order universe, such as Chaldea, and how he navigates through the politics and power dynamics.
A crossover with the Naruto universe could also be exciting. Imagine Servants like Archer using their skills against ninjas. Naruto characters might be intrigued by the concept of Servants and their powers. It could lead to a story where they exchange combat techniques and learn from each other.
One idea could be creating an OC who is a time - traveling mage from a future where the Grail Wars have had catastrophic consequences. This OC could be on a mission to change the past to prevent the disasters. Another idea is an OC that is a hybrid of a Servant and a human, with unique abilities and a complex backstory that ties into the existing lore of Fate Grand Order. Maybe an OC who was experimented on by the mages in Chaldea and gained strange powers as a result.
Another plot could involve Naruto being mistaken for a new type of heroic spirit. He then has to prove himself in the Grail War while also trying to find a way back to his own world. Along the way, he could meet some of the Fate characters and learn about their backstories, which would be a great way to combine the two universes.
To write a good character arc for Emiya in fanfiction, you need to explore his past traumas. He has seen a lot of death and destruction. You can make him face similar situations again, but this time, he has the chance to change the outcome. Let him grow from being a lone wolf to someone who can rely on his allies more. Show his internal struggle as he tries to balance his old self - reliant ways with the new need to trust others. This development can be gradual, with small events leading to a big change in his character by the end of the story.
One plot idea could be that Naruto is summoned into the Fate Grand Order universe. He teams up with Mash to fight against the various Servants who have gone berserk. Naruto's unique jutsu like Rasengan could be a new and powerful attack in that world.
You can start by looking on fanfiction platforms like Archive of Our Own (AO3). It has a large collection of Fate Grand Order fanfictions featuring Shirou Emiya. Just use the search function and type in 'Shirou Emiya Fate Grand Order' to get relevant results.
One interesting concept could be the exploration of different Servant backstories in more depth. For example, delving into the pre - summon experiences of a Servant that might not be fully covered in the game. Another could be alternative versions of the Holy Grail Wars, with different rules or participants. Also, the fanfiction might explore the relationships between the Masters and Servants in unique ways, like how they interact outside of battle situations.
One interesting concept could be the combination of different character abilities from Fate Grand Order and RWBY. For example, seeing how a Servant's Noble Phantasm might interact with a RWBY character's semblance. Another concept could be the crossovers in the storylines, like how the overarching plot of Fate Grand Order, which often involves saving the world from various threats, could mesh with RWBY's fight against the Grimm. Also, the character relationships that form between the two universes' characters can be really engaging, such as a Fate character forming an unexpected alliance with a RWBY team member.