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.
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.
They blend the universes by bringing together the unique magic systems. In Kingdom Hearts, we have the Keyblade and Heartless, while Once Upon a Time has its own brand of magic like spells cast by the Evil Queen. Fanfictions often have characters from one universe encountering these elements from the other. For instance, a character from Kingdom Hearts might find themselves in the Enchanted Forest and have to deal with a magical curse.
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.
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.
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.
There's a fanfiction called 'Kingdom Hearts: Pokemon Edition'. It takes elements from Kingdom Hearts like the Heartless and mixes them with the concept of Pokemon. The story follows a group of trainers who discover a connection between the two worlds and must fight to protect both. It has a really interesting plot that keeps you hooked from start to finish.
I really like 'Sora's Time - Travel Mishap'. It's a fun fanfiction where Sora somehow gets sent back in time. He has to deal with the consequences of changing the past while still trying to get back to his own time. It has some really cool interactions with younger versions of the characters we know. 'Sora's Hidden Power' is also a good read. It explores a new power that Sora discovers within himself, and how he uses it to fight against a new, powerful enemy.
A really good 'Once Upon a Time' fanfiction is 'The Dark One's Daughter'. It has a unique take on Rumpelstiltskin's family. The story is filled with mystery and drama. Another one is 'Hook's Hidden Past' which uncovers more about Captain Hook's life before he became the infamous pirate we know. 'The Fairies' New Magic' is also great as it shows the fairies in a new light, with different magical abilities and a whole new set of challenges they face.
One great fanfiction is 'The Dark Side of Neverland'. It delves deep into the complex relationship between Peter Pan and Felix. It shows how Felix is extremely loyal to Pan and the lengths he would go to protect Pan's rule in Neverland. The story has a great deal of adventure and mystery, making it a thrilling read.
There's also 'Regina's Redemption through Archie'. In this fanfiction, Archie becomes a crucial figure in Regina's journey towards redemption. It shows how his kind and understanding nature can influence her to make better choices. It's really interesting to see their interactions unfold as the story progresses.