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.
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 interesting plot could be that the ponies from MLP accidentally find themselves in the Kingdom Hearts universe. They might mistake the Heartless for some kind of dark magic creatures from their own world and try to fight them using their magic. For example, Twilight Sparkle could use her powerful spells to combat the Heartless while trying to figure out how to get back to Equestria.
One idea could be that Sora and his friends end up in Equestria. They could team up with the ponies to fight a new threat that is affecting both their worlds. Maybe the Heartless are somehow being drawn to Equestria's magic and they need to find a way to stop it together.
The idea of friendship is also quite common. Since MLP is big on friendship, fanfictions often use that concept within the context of Kingdom Hearts' more battle - and adventure - oriented world. For example, the ponies' friendship might be a source of power against the darkness in Kingdom Hearts. And also, the concept of different worlds is shared. Just as Kingdom Hearts has various Disney - inspired worlds, MLP's Equestria can be seen as another world in the Kingdom Hearts universe in these fanfictions.
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.
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.
It could also be a new character created specifically for the fanfiction. This character could be from the world of the crossover and have some special ability or knowledge that helps in the fight against the Heartless. Maybe they have a deep understanding of the magic or technology in their world that can be used to counter the Heartless threat.
Twilight would be excited to explore a new fictional universe. She'd start taking notes on the magic systems and different worlds in the fanfiction. Rainbow Dash would be bored if there wasn't enough action. She'd be looking for epic battles and heroic deeds. Pinkie Pie would love the fanfiction just for the fun of it, no matter what it was about.
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.