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.
Your OC could be a healer from a peaceful world. In Kingdom Hearts 3, there are many battles and injuries. The OC could be drawn into the main conflict because they sense a great imbalance in the world. They would have to learn to fight while also using their healing powers. They might develop a special bond with a character like Kairi, who also has a role related to balance and healing in the game.
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.
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.
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.
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.
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 an OC who is a keyblade wielder from a long - lost world. They meet Ventus during his travels and join him on his journey to restore the balance in different worlds. This OC could have a unique keyblade design inspired by their home world's culture.
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 idea could be that the characters from Kingdom Hearts have to team up with Fairy Tail wizards to defeat a new powerful dark force that threatens both their worlds. This dark force could be stealing the keyblades' power and the magic of Fairy Tail. Another plot could involve a character from Kingdom Hearts getting lost in the Fairy Tail world and having to learn magic to find their way back home while also helping Fairy Tail with their own problems, like fighting a guild that has turned evil. And a third plot could be about a Fairy Tail wizard finding a keyblade and having to learn how to use it with the help of Sora and his friends to stop an ancient evil that is starting to awaken in both worlds.
Another interesting concept is an OC that has a special connection to the darkness but is trying to fight it. This OC could start as a character who is feared by others in the 'Kingdom Hearts' universe because of their association with the dark side, but as the fanfiction progresses, they prove their worth and loyalty to the light. Maybe they were experimented on by the villains, which gave them this connection to the darkness.
One idea could be that the Heartless somehow find their way into the Marvel universe and start attacking various locations. The Avengers, being the protectors, team up with Sora and his friends from Kingdom Hearts. They initially have some clashes due to different fighting styles and ways of understanding the world, but eventually learn from each other and combine their powers to defeat the Heartless. For example, Iron Man could use his technology to analyze the Heartless' weaknesses, while Sora uses his Keyblade to directly combat them.