There might also be some secondary characters who become more prominent in the crossover. Maybe a character who was overlooked in their original kingdom but has a special skill or knowledge that becomes crucial in the new, crossover setting. This can make them a fan favorite as they get to shine in a different context.
Often, the main protagonists from the original kingdoms tend to be popular. For example, if there are heroic characters known for their bravery in their respective kingdoms, they will likely be central in the crossover fanfiction as well.
Well, one interesting concept could be the unexpected character interactions. For example, characters from different kingdoms might have to work together in a way that defies their normal allegiances in the original stories. This can lead to unique storylines.
Aqua is popular as well. Her loyalty and her own journey in 'Kingdom Hearts Birth by Sleep' make her an interesting character for crossovers. She could be paired with characters from different franchises in fanfiction. For example, if she were to cross over with a character from a magical girl anime, her strong will and magical abilities could lead to some really engaging storylines.
Sora and Cloud are quite popular. Sora's cheerful and determined personality combined with Cloud's brooding and powerful presence make for interesting interactions. They can be portrayed as allies, learning from each other's fighting styles, or even as rivals in some fanfictions.
Uhtred is often a very popular character in fanfiction. His complex character, being a Saxon raised by Danes, offers a lot of room for different storylines.
Sora is often very popular in Kingdom Hearts fanfiction. He's the main protagonist in the game, so many fans like to write about his further adventures or different versions of him. Another popular character is Riku. His complex relationship with Sora and his own journey with light and dark make him a fascinating subject for fanfiction writers.
Sasuke is also a top pick. His brooding nature and complex backstory make him a great character to delve deeper into. In fanfiction, we might see him in different roles, such as a reluctant leader or a mentor to younger ninjas.
Arthur, without a doubt, is often a very popular character. He's the main character in the original, so many fanfictions will center around him, his adventures, and his growth throughout the story.
Judy Hopps from 'Zootopia' is a great character for crossovers. She's smart, determined and a great detective. She could easily fit into many other animal kingdom stories. Another good one is Simba from 'The Lion King'. His journey from a cub to a king gives a lot of room for crossovers. He could meet other characters and share his wisdom or learn from them.
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.
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.