Maybe the OC is a being from the Realm of Darkness, but not evil. They have been trying to find a way to cross into the Realm of Light without being corrupted. The OC meets Riku and they form an unlikely alliance. They journey through different worlds, facing Heartless and Nobodies, while the OC tries to resist the pull of the Darkness within them. Along the way, they learn about each other's pasts and grow as characters.
One idea could be that the OC is a keyblade wielder from a forgotten world. They are discovered by Sora and his friends during their travels. The OC has a unique keyblade with powers that are tied to a long - lost magic. This could lead to adventures where they have to unlock the secrets of this magic together to defeat a new threat in the Kingdom Hearts universe.
The OC could be a young mage who has a special connection to the stars. In the Kingdom Hearts world, this connection gives them the ability to see things others can't, like hidden paths in different worlds or the true forms of certain enemies. They join forces with Kairi and together they use their combined powers - Kairi's light and the OC's star - based magic - to help protect the worlds from a new kind of enemy that is using the power of the stars in a negative way.
Perhaps the OC has a special power that is linked to Ventus' past in some way. For example, she can see glimpses of his forgotten memories. This leads to a journey where they explore his past together, while also growing closer and falling in love along the way. They might face challenges from Organization XIII who are interested in the OC's power.
Another interesting concept is an OC who is a member of an organization that opposes Organization XIII in a different way. Maybe they use magic that is not based on darkness or light, but something in between, which causes them to be shunned by both sides at first.
One idea could be creating an OC who is a keyblade wielder from a forgotten world. This OC could be on a quest to discover the secrets of their world's connection to the greater Kingdom Hearts universe. They might encounter classic characters like Sora and Riku and have to prove their worth as a keyblade wielder.
Well, you could write a fanfiction about a group of keyblade wielders who get trapped in a strange, distorted version of their own world. They have to figure out how to escape while also dealing with their own personal demons and secrets. As they explore this new and dangerous place, they learn more about themselves and the true nature of their powers, which could lead to some interesting character development.
One idea could be a crossover where the keyblade wielders from Kingdom Hearts end up in the world of Blade. They could team up with the Blade warriors to fight a new threat that is a combination of Heartless and the vampires from Blade. Maybe Sora discovers that his keyblade has new powers when used against the vampires.
Perhaps there could be a plot where the Organization XIII members from Kingdom Hearts have an interest in the Relics from RWBY. They start to manipulate events in the RWBY world to try and get their hands on these powerful artifacts. The RWBY characters, suspecting something is amiss, team up with Donald and Goofy who have also arrived in their world. Together, they have to figure out the Organization XIII's plan and stop them before they can steal the Relics.
One idea could be that the main character from 'The Gamer' gains the power to level up in the 'Kingdom Hearts' world. So they start leveling up their keyblade skills like it's an RPG. They could also gain stats related to magic, speed, and strength, which would change how they interact with the Heartless and Nobodies.
Maybe there could be a story where the Keyblade wielders from Kingdom Hearts visit the world of FFXV to learn about their unique form of magic. They could then exchange knowledge, like how Noctis' warp strikes could be adapted into a new Keyblade ability.
One idea could be creating an OC who is a keyblade wielder from a forgotten world consumed by darkness. This OC could be on a journey to find a way to restore their world while also struggling with the allure of the darkness within them.
One idea could be exploring the backstories of the new characters introduced in Kingdom Hearts 3 in more depth. For example, you could write about how they came to be in the different worlds and what their true motives are.