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.
A new character could be introduced as the main character. This character could be a native of the kung fu - inspired world in the crossover. They could be the one to teach Sora and his friends about kung fu, and then join them on their journey to fight against the darkness that threatens both worlds.
Sora would be an obvious choice as he has been the central character throughout the series. His journey doesn't end with the defeat of Xehanort, and there are still many things he could experience in the aftermath.
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.
One interesting concept could be the exploration of different worlds colliding. For example, the Heartless from Kingdom Hearts might enter a world from another franchise in the crossover. This could lead to unique battles and interactions between characters that wouldn't happen in the original Kingdom Hearts universe.
Female Heartless can add a new dimension to Kingdom Hearts fanfiction. They can break the mold of the traditional view of Heartless as just mindless monsters. For example, a female Heartless could be more in - touch with emotions, even if they are distorted as a Heartless. This could lead to interesting character dynamics, like a love - hate relationship with a keyblade wielder.
In a Kingdom Hearts female Heartless fanfiction, the female Heartless could have unique abilities. She could use her femininity to her advantage, perhaps having a more stealthy or alluring approach compared to male Heartless. The story could explore how she uses these abilities in her interactions with other characters. For instance, she might be able to charm some of the lesser enemies to do her bidding while she tries to figure out her place in the Kingdom Hearts universe.
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.
In this type of fanfiction, the dynamic between Sora and the Heartless can be really interesting. Some fanfics might focus on the power struggle. Sora has his own set of powers, and the Heartless are known for their menacing nature. Their interaction could lead to some intense battles or unexpected alliances. It all depends on how the author wants to portray this relationship within the Kingdom Hearts universe.
Hermione would be a great character to focus on. She is intelligent and curious. In a fanfiction involving the Heartless from Kingdom Hearts, she would be the one who would immediately start researching about them. She would try to find any connections between the magic in the Harry Potter world and the existence of the Heartless. Her thirst for knowledge would lead her to make some interesting discoveries and she could potentially come up with unique ways to combat the Heartless using magic spells and potions.
A possible main antagonist could be a corrupted version of Bellwether. After her time in prison, she could find a way to tap into the power of darkness. She wants revenge on Zootopia and uses the Heartless to cause chaos. She would be a devious antagonist as she already knows the ins and outs of Zootopia.