Definitely Leo. Since he is the main character, a lot of fanfic focuses on him. Writers like to put him in different situations and explore his character further. Another character that gets a lot of attention is Zapp. His brash and bold personality gives writers a lot to work with. He can be put in comical situations or serious battles. And Black is also a popular choice. His past and his relationship with White are often delved into in fanfic.
For Kekkai Sensen, Leo is an interesting character to focus on. His abilities and his role in the story of Kekkai Sensen could be explored further in a fanfiction with Naruto elements. He could have some really cool interactions with Naruto characters.
In the 'Kekkai Sensen' manga yaoi, you could expect to see deep character development of the male pairings, along with complex relationship dynamics. Also, the art style might play a significant role in enhancing the romantic atmosphere.
White and Kaneki from 'Tokyo Ghoul'. White's power and her connection to Leo could play an interesting role when paired with Kaneki's ghoul powers. They could both struggle with their dual identities in a crossover scenario, and perhaps find a way to coexist or fight against a common enemy together.
One popular theme is the exploration of the unique abilities of the characters. For example, how they use their powers in different situations not shown in the original work. Another theme could be the relationships between characters. Whether it's the deepening of friendships or the development of romantic interests. Also, alternate universe scenarios are quite common, like a world where the kekkai sensen society operates in a completely different way.
One idea could be exploring the backstories of the minor characters in Kekkai Sensen. For example, you could write about the life of a random citizen in Hellsalem's Lot and how they deal with the chaos. Another idea is to create an alternate universe where the main characters have different powers or personalities. Maybe Leo has the power to control time instead of his usual ability.
One idea could be a story where Chain and Leo have to team up to solve a mystery in the hidden world of Hellsalem's Lot. They might start off not getting along well due to their different personalities, but as they face various challenges together, they gradually learn to trust and rely on each other.