Perhaps Riku could be mistaken for a dark villain in the Once Upon a Time world. He then has to prove his good intentions while also helping the characters there understand the concept of the Heart and Darkness. He might find an unexpected ally in Regina, who sees a bit of herself in his struggle.
One idea could be that Sora and his friends get transported to the Once Upon a Time realm. They could team up with Emma Swan to fight against the Heartless that start to invade Storybrooke. Maybe the Keyblade can be the key to unlocking some of the dark secrets in the town's past.
Another plot idea is a crossover of the villains. Imagine Maleficent from 'Kingdom Hearts' teaming up with the Evil Queen from 'Once Upon a Time'. They could combine their powers and knowledge to try and take over all the worlds. The heroes from both universes would then have to work together, which would lead to some really interesting character interactions and battles.
A really popular one is 'Heart of the Enchanted Forest'. This fanfic delves deep into how the Heartless could potentially invade the Enchanted Forest from Once Upon a Time. It has some great action scenes with characters from Kingdom Hearts fighting alongside the heroes of Once Upon a Time. For example, Riku teams up with Prince Charming to battle a horde of Heartless that are threatening the forest.
One idea could be exploring the backstories of the lost boys in more detail. Maybe one of them has a hidden connection to a character from the outside world. Another plot could involve a new villain coming to Neverland who has a different kind of magic that challenges Peter Pan. Also, a story about Wendy's granddaughter returning to Neverland to find a lost family heirloom could be interesting.
Sora and Henry. Henry is always looking for adventure and has a connection to the storybook world. Sora could show him more about different worlds and how to fight using the Keyblade. They could form a really cool mentor - mentee relationship.
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.
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.