One idea could be exploring what would happen if Ainz made an alliance with an unexpected race in the New World. For example, forming a bond with a group of dragon lords and how that would change the power dynamics.
Another plot idea is to focus on the daily lives of the NPCs in Nazarick when Ainz is away on an extended mission. How do they maintain order and deal with internal conflicts? This can show a different side of the Overlord world.
You could also write a fanfic about a character from the New World who manages to infiltrate Nazarick undetected for a period of time and what they discover during their stay. It would add an element of mystery and suspense to the story.
A third idea is a fusion of the two worlds. The shinobi villages and Nazarick could form a trade or knowledge - exchange system. Characters from both sides would have to learn about each other's cultures and abilities. This could lead to some really fun interactions, like Albedo trying to learn ninjutsu from Kakashi, or Sasuke being intrigued by the power of the Supreme Beings in the Overlord world.
One interesting plot could be exploring the backstories of the minor characters in the Overlord game world. For example, the lives of the villagers who interact with the powerful overlord character.
One plot idea could be that a character from RWBY gets transported to the world of Overlord. They might have to navigate the power - hungry and often dangerous world, trying to find a way back home while also dealing with the various powerful beings in Overlord. For example, Ruby could end up in the Tomb of Nazarick and have to figure out how to interact with the Floor Guardians without getting killed.
You could do a fanfic for a survival game plot. Imagine the game is set in a post - apocalyptic world. In the fanfic, the main group of survivors find an abandoned military bunker that seems like a haven at first. But it turns out to be filled with traps and secrets. They have to use their wits and the skills they've developed in the game to escape and find a new place to call home.
One idea could be to explore an alternate timeline where the main characters make different choices at a crucial moment. For example, if a character who usually chooses to fight decides to negotiate instead, how would that change the course of the story? Another plot could involve a new character being introduced who has a special power that disrupts the existing power balance in the 'game of fate' world. This new character could either be a force for good or evil, depending on their motives.
One idea could be a power struggle within the Sunagakure village. Maybe a young shinobi discovers a long - hidden secret about the village's leadership that throws the existing power structure into chaos. Another could be a story centered around a group of shinobi from Sunagakure who are on a mission to retrieve a powerful and ancient artifact that has been lost outside the village for centuries. It could also involve a love story between two shinobi from different clans in Sunagakure, but their relationship is complicated by the long - standing rivalries between their clans.
A great plot for 'the rune game fanfic' might be centered around a young novice who stumbles upon a broken rune. They then set out on a quest to find the missing pieces and in the process, learn about the history of the runes, make new friends, and face off against other players who also want the complete rune for their own, more sinister purposes. As they travel through different regions of the game, they encounter various challenges that test their skills and knowledge of rune magic.
One idea could be to explore an alternate universe where the characters have different semblances. For example, Ruby could have a semblance that allows her to manipulate time briefly instead of her speed. Another plot could focus on a new villain in the game's world who has the power to corrupt semblances, forcing the RWBY team to find a way to counter this new threat without relying solely on their usual powers. Also, a story about the backstories of the minor characters in the 'rwby the game' could be really engaging.
One idea could be to create an alternate origin story for the main character in Spy Game. Maybe they were initially a hacker who got recruited into the spy world after accidentally uncovering a major conspiracy. Another idea is to have a cross - over with another popular spy franchise, where characters from both worlds have to team up to take down a common enemy.
One idea could be a story about a soldier who discovers a hidden power within the game world and has to learn to control it while facing off against powerful enemies. Another could be a soldier's journey to reunite with their lost comrades within the game. Maybe a story where a soldier is wrongly accused of cheating in the game and has to prove their innocence.