One common theme is power dynamics. The OC often has to navigate their relationship with powerful servants, who may have different levels of loyalty or attraction. For example, in some fanfics, a servant might initially be dismissive of the OC but later become part of the harem as they see the OC's growth or hidden strength.
One popular Fate Grand Order OC Harem fanfiction is 'The Summoned's Journey'. It has an interesting plot where the original character gets involved with multiple servants in a unique harem situation. The character development is well - done, and it keeps you engaged throughout.
Jeanne d'Arc is also frequently seen in such fanfictions. Her noble and kind nature makes her a great addition to Ritsuka's harem. And then there's Scathach, with her powerful and mysterious persona, she can bring a lot of interesting dynamics to the harem in the fanfiction.
A common trope is the 'protective harem'. In Fate Grand Order harem fanfiction, the Servants in the harem often feel the need to protect the Master (the protagonist). This can lead to some interesting scenarios where they compete to be the best protector. Also, the 'unexpected bond' trope is quite popular. It's when two characters who seem very different at first end up having a deep connection within the harem. For instance, a heroic Servant and a more villainous - like Servant might form an unexpected bond over their shared love for the Master.
Well, these kinds of fanfictions often explore different aspects of the Servant - Master relationship. Since it's an SI OC Servant, the story can take a very personal turn. The author can play with the idea of how a modern - day person, if inserted into the FGO world as a Servant, would react to the historical and mythological figures around them.
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.
Perhaps a plot where Ritsuka's harem has to work together to defend a certain area from powerful enemies in the 'Fate Grand Order' universe. The harem members might have different strategies based on their unique abilities, and Ritsuka has to manage them all.
First, you need to be familiar with the Fate Grand Order universe. Know the characters, their backstories, and their abilities. Then, decide on the main character, usually the Master. Next, think about the relationships you want to develop in the harem. You can start with a simple idea like a new Servant arriving and how they interact with the existing ones in the harem. Also, consider the plot, whether it's about saving the world or just the daily adventures in Chaldea with the harem.
One of the things that makes it special is the freedom to create. Writers can design their own Servant with any kind of powers, appearance, and personality. It's like creating a brand - new character in the rich world of Fate Grand Order.
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.