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 a plot where the keyblade wielders are summoned to the Queens Blade world because there is a strange darkness emerging that is similar to the Darkness in Kingdom Hearts. Kairi could form a special bond with one of the Queens Blade heroines. They would then work together to figure out the nature of this darkness. The story could explore how the magic and power systems of both worlds interact and how the characters adapt to being in a new and strange world.
One idea could be a crossover where the characters from Kingdom Hearts find themselves in the world of Blade. They could team up with the Blade warriors to fight against a common enemy, like a new form of Heartless that has emerged in the Blade universe. This would lead to interesting interactions as they learn about each other's powers and worlds.
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.
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.
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.
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 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.
One interesting storyline could be Sora and Blade teaming up to face a new, powerful enemy in a world that combines elements from both their universes. They might have to learn to combine their unique combat styles to defeat this foe.
Perhaps a fanfiction where Ventus has a premonition of a great danger that only he and Vanitas can prevent together. As they journey to stop it, they gradually form an unexpected bond. They learn that their connection is deeper than they initially thought, and this realization changes their relationship throughout the story.