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.
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.
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.
Another interesting plot could be Loki falling in love with a character from Once Upon a Time. Maybe he has a relationship with a princess or a powerful sorceress, and their love story is filled with magic, drama, and challenges as they come from different worlds.
One idea could be a new agent arriving at Warehouse 13 with a mysterious past that turns out to be linked to an artifact that was thought to be lost forever. This new agent might unknowingly hold the key to controlling the artifact's power, causing a stir among the existing agents as they try to figure out how to handle the situation.
A love - interest plot for Gideon would also be interesting. He could fall in love with a character from a different realm within the Once Upon a Time universe. Their relationship would be complicated by the different rules and cultures of their respective realms. They would have to overcome many obstacles, like disapproving family members or the threat of war between their two realms.
One idea could be exploring Gothel's backstory before she became the villain we know. Maybe she had a tragic love in her youth that turned her bitter. Another plot could involve Gothel teaming up with an unexpected character from the Once Upon a Time universe, like a hero who thinks they can reform her. A third option is to create a story where Gothel has a daughter of her own and we see how she raises her in contrast to how she treated Rapunzel.
One idea could be to create a story where a new character, an aspiring actress, befriends Rick Dalton and Cliff Booth. She gets involved in their crazy Hollywood adventures and helps Rick revive his career in an unexpected way, like by introducing him to a new type of movie role that suits his acting style better.
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.
Sora would be really interesting. He has such an innocent and determined nature in 'Kingdom Hearts'. In the 'Once Upon a Time' setting, he could be the one to bring a sense of hope and adventure. His reactions to the different fairy - tale characters and settings would be fun to explore.