The interaction between the characters could be really interesting. Ariel, being a mermaid, has a very different way of life compared to Sora. Sora is used to traveling between different worlds and fighting enemies. When they meet, Ariel's naivete and Sora's experience would create an interesting dynamic. For example, Ariel might not understand the full danger of the Heartless at first, and Sora would have to explain it to her. Meanwhile, Sebastian might try to organize everyone like he does in his own underwater world, which could lead to some comical situations when dealing with the more laid - back Goofy and the sometimes hot - headed Donald.
Ariel's friends, like Flounder and Sebastian, would be a bit wary of Sora, Donald, and Goofy at first. But as they see that they are there to help, they would warm up. Sora would try to communicate with them in his own kind way. Ariel would probably want to learn some fighting moves from Sora to defend herself against the Heartless. Sora would teach her some basic self - defense using the Keyblade as an example. And Donald and Goofy would share their stories from their adventures in other worlds with Ariel and her friends.
Sora would likely be very protective of Ariel. He would see her as someone in need of help, just like he does with many characters in different worlds. Ariel, on the other hand, would be fascinated by Sora's world and his Keyblade. She might ask him a lot of questions. Donald could get frustrated with Ariel's overly curious nature at times, while Goofy would just be friendly and go along with whatever is happening.
One possible plot could be that Sora and his friends land in the underwater world of The Little Mermaid. Ariel could be in trouble, perhaps captured by the Heartless. Sora, with his Keyblade, would fight to free her. Donald and Goofy could help by using their magic and strength respectively. They would then have to find a way to restore the underwater kingdom's heart which has been affected by the Heartless presence.
The Keyblade wielders like Sora, Riku, and Kairi would bring a sense of adventure and a different kind of magic to Equestria. The ponies, on the other hand, could teach them about the power of friendship in a more direct way. Donald and Goofy might be a bit startled by the ponies at first but would soon get used to them. Donald's magic could be compared to the magic of some of the unicorn ponies, and they could exchange magical knowledge.
One idea could be to explore what would happen if Sora and his friends got trapped in Ariel's underwater world during a Heartless attack. They'd have to team up with Ariel and her friends to fight off the Heartless while also dealing with Ursula's schemes in a new way.
Ursula is an obvious choice as she is the main villain in the original Little Mermaid story. In a Kingdom Hearts fanfiction, she could be even more powerful, perhaps having made a deal with the Heartless or some other dark force to gain more control over the underwater world and other worlds connected to it.
One great 'Kingdom Hearts Crossover Fanfiction' is 'Kingdom Hearts and Final Fantasy: The Convergence'. It combines the elements of Kingdom Hearts and Final Fantasy in a really interesting way. The characters from both universes interact in a believable manner, and the plot keeps you hooked from start to finish.
Another aspect could be how Sora's presence affects the balance of power in the My Little Pony world. His powers are quite different from what the ponies are used to. The villains in the My Little Pony world might see Sora as a new target or a threat. Meanwhile, the ponies might look up to Sora as a new hero, and his actions could inspire them to be braver and more adventurous in protecting their own world.
Aqua would be a really interesting crossover character with Roxas. Aqua has her own strong powers and a complex backstory. When she meets Roxas, she could see something in him that reminds her of Ventus. This could lead to a relationship where she tries to help him while also learning more about herself. Also, Riku would be a good fit. Riku has had his own journey of redemption and self - discovery. His experience could be useful for Roxas in a crossover situation, and they could have some intense battles together against the forces of darkness.
Riku could also play a major role. His more complex and sometimes brooding personality could add an interesting dynamic. He might start off as a bit of an outsider in Equestria but eventually come to understand and care for the ponies, especially if there are some ponies who remind him of his friends from his own world.
First, you need to have a solid understanding of both universes involved. For example, if you're doing a Kingdom Hearts and Pokemon crossover, know the Pokemon types, their abilities, and how they interact. Also, be familiar with Kingdom Hearts' keyblade mechanics and the different worlds. Then, create a unique plot that ties the two together. Maybe the Pokemon are new forms of Heartless or something.
Ruby's speed and scythe - wielding skills could be enhanced by the Keyblade's magic. For example, she could use the Keyblade's power to make her Crescent Rose even more powerful, like adding elemental attacks. Sora's ability to quickly adapt to new situations and his combat skills with the Keyblade could be combined with the Huntsmen training in RWBY. He could learn new fighting styles from the RWBY characters and they could learn from his experience with different enemies. Donald and Goofy's support abilities could be used to assist the RWBY teams in battles, like Donald's magic for healing or Goofy's defense - boosting skills.