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.
Perhaps the 'The Gamer' system allows the character to create their own unique summons. In 'Kingdom Hearts', summons are important allies. So, the character could use their gamer knowledge to combine different elements from the 'Kingdom Hearts' world to make super - powerful summons. They might use the power of light and dark in a new way, something that the original characters in 'Kingdom Hearts' have never thought of.
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 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.
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.
A cool plot could be that a new villain emerges, who has the power to manipulate both time (like in Chrono Trigger) and hearts (like in Kingdom Hearts). The heroes from both universes have to combine their skills and knowledge. For example, Sora's ability to unlock hearts and Crono's time - manipulation skills could be used together to defeat this powerful foe. They would need to journey through various worlds from both franchises, facing challenges specific to each world's time and heart - related mysteries.
A common plot idea is the crossover of the magic systems. For example, how the magic from Twisted Wonderland would interact with the Keyblade magic in Kingdom Hearts. Maybe the characters from Twisted Wonderland could learn to use Keyblades or the Kingdom Hearts characters could adapt to the unique spells in Twisted Wonderland.
One idea could be that the reader is a new student at the Kingdom Hearts School. They're lost on their first day and a popular character from the series helps them find their way. This could start a friendship or even a budding romance.