You could write a fanfiction where Peter Pan decides to leave Neverland and join Sora on his journey through the different worlds in Kingdom Hearts. This would bring a whole new dynamic to the group. Peter's carefree and adventurous nature might clash with some of the more serious characters, but at the same time, his skills and knowledge of magic could be a great asset. He could also introduce Sora and the others to new areas in Neverland that they haven't explored yet, which might hold secrets related to the Heartless or the overall plot of Kingdom Hearts.
Well, you could write a fanfiction where Peter Pan and Wendy grow up but still remember their adventures in Neverland. They could try to recreate that sense of magic in the real world, perhaps by opening a place where children can experience a bit of Neverland. Or, what if a new villain emerged in Neverland and Peter and Wendy had to team up in a different way than before to defeat them? They might have to use Wendy's more grown - up thinking and Peter's knowledge of Neverland's secrets.
Captain Hook would be an obvious choice as the main villain. He already has a long - standing feud with Peter Pan in the original story. In a Kingdom Hearts fanfiction, he could team up with the Heartless or be influenced by the Darkness to become even more menacing. He could also be after the power of the Keyblade, thinking it would give him an edge over Peter and his friends.
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 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 a fanfiction where the characters from Kingdom Hearts watch their own adventures from an alternate universe. It would be really cool to see their reactions to different versions of themselves and the events that unfold.
One idea could be a crossover where Sora and his friends from Kingdom Hearts end up in the Pokemon world. They could team up with trainers to battle against the Heartless that have somehow invaded the Pokemon realm. Another idea is that some Pokemon could have the power of Keyblades, and they use it to protect their regions from both Pokemon villains and the threats from Kingdom Hearts like Organization XIII. Maybe a Pokemon like Pikachu could befriend Sora and they go on an adventure together to find a way to get Sora back to his own world while also helping the Pokemon world.
One idea could be a story where the Pokemon trainers are transported to the Kingdom Hearts universe and have to team up with Sora and his friends to fight against the Heartless. They could use their Pokemon's unique abilities in combination with keyblades to defeat the enemies.
Well, you could write a fanfiction where Peter Pan loses his ability to fly and Wendy has to help him find a way to regain it. It could involve them going on a journey across Neverland, facing various challenges and meeting new characters along the way. They might have to seek the help of the mermaids or the Lost Boys. This would not only test their friendship but also show their growth as characters.
Another idea is a crossover with The Chronicles of Narnia. Peter could enter Narnia through some magical portal. He'd interact with the talking animals and the Pevensie children, and his skills in fighting pirates could be useful in Narnia's battles.
Perhaps in the fanfiction, Peter Pan gets injured during one of his battles with Captain Hook. Jane, who has learned some basic first - aid skills from her own world, takes care of him. This leads to a deeper bond between them, and Peter starts to see Jane in a new light, not just as a visitor to Neverland but as a true friend and an equal.