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.
An interesting concept for Fate Grand Order OC fanfiction is an OC who is a detective - like figure in the world of the game. They could be investigating strange occurrences related to the Servants that are not part of the main story quests. For example, disappearances of minor Servants or strange magical phenomena in areas not yet explored in the game. This OC would use their own set of skills, perhaps some magic - infused detective skills, to solve these mysteries. Also, an OC that is a wandering bard who travels from singularity to singularity, documenting the tales of the Servants and the Grail Wars in song and story could be really cool.
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 interesting story could be about an original character (OC) who has a unique ability and gets involved in the complex plot of Fate Grand Order. For example, an OC with time - controlling powers that changes the outcome of a certain battle.
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.
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.
One interesting Fate Grand Order OC Servant fanfiction could be about a new Servant with the power of time manipulation. This Servant could be from a lost era and has unique abilities that change the course of battles in the Holy Grail War. Their backstory might involve a tragic past related to the misuse of time magic, which makes for a great plot device.
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.
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.
A possible plot could be Scathach's exploration of the modern world. Since she is from an ancient time, her reactions to modern technology, culture, and society can be a great source of interesting storylines. She might be initially confused by things like smartphones or cars, but then gradually learn to use them in her own unique way. This could also lead to humorous situations, like when she tries to order food at a fast - food restaurant for the first time.
Her motherly nature. In many fanfictions, Raikou's motherly side is explored, especially in relation to other servants or the Master. She can be very protective.