Riku and Hook. Hook is a bit of a rogue, and Riku has had his own struggles with the darkness. They could bond over their past mistakes and experiences. Hook could show Riku some of the pirate ways in the Once Upon a Time realm, while Riku could teach Hook about the power of the heart and how to fight against dark forces more effectively.
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.
Sora and Spider - Man would be a really cool pairing. Sora has his acrobatic skills with the Keyblade, and Spider - Man is all about agility and using his webs in combat. They could have a great time swapping stories about their different worlds and learning new moves from each other. For instance, Sora could teach Spider - Man some basic Keyblade forms, and Spider - Man could show Sora how to use his environment better for combat, like swinging from buildings.
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.
Another interesting pairing could be Sakura and Kairi. Sakura is a strong - willed kunoichi who has grown from being a bit of a fangirl to a capable fighter. Kairi is also a strong - hearted girl. They could bond over their experiences of being in the background sometimes but still being important to their respective groups. They might even learn some new self - defense moves from each other.
Ichigo and Sora would be an obvious and interesting pairing. Ichigo is a strong - willed Soul Reaper and Sora is a determined Keyblade wielder. Their different fighting styles and personalities would create great chemistry. They could learn from each other, like Sora teaching Ichigo about the different worlds he has visited, and Ichigo showing Sora how to better handle spiritual energy.
Ruby and Sora would be a really interesting pairing. Ruby is young and full of energy, always eager to help and fight for what's right. Sora is the same, with his big heart and determination. They could learn a lot from each other, like Ruby could show Sora some of her cool scythe techniques and Sora could teach Ruby more about using the Keyblade effectively.
Sora and Elsa would be really interesting. Sora, with his positive and adventurous spirit, could help Elsa come out of her shell more. Elsa, with her powerful ice magic, could offer Sora a different kind of magic to understand and work with.
Aqua and Ignis could also be an interesting pair. Aqua's strong magical abilities and discipline as a Keyblade Master could be compared and combined with Ignis' strategic mind and combat skills. They might work together to solve complex problems that require both magic and strategy.
One of the most interesting pairings could be Yhwach and Ansem. Yhwach has his goal of changing the world in Bleach, and Ansem is obsessed with the power of darkness in Kingdom Hearts. They could form an alliance based on their mutual desires for power. Also, Tatsuki and Naminé. Tatsuki is a strong - willed human in Bleach, and Naminé has her special abilities in Kingdom Hearts. They could develop a friendship where Tatsuki protects Naminé while Naminé uses her powers to help Tatsuki understand the strange new world she's in. And Renji and Terra. Renji with his loyalty to his friends and his zanpakuto skills, and Terra with his struggle between light and dark and his Keyblade mastery, would make for a great pair.
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.