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.
The Kekkai Sensen anime is known for being quite faithful to the manga. It maintains the original story and characters well. Though there are some creative adjustments made to make it more appealing on screen, they are mostly in line with the manga and fans tend to like them.
Well, a great plot could be Leonardo having to face his own inner demons in a very literal sense. In the fanfiction, his fears and insecurities could manifest as physical entities that he has to battle. This would not only be a test of his physical strength but also his mental and emotional stability. And also, perhaps a plot where Leonardo discovers a hidden power within himself that he has to learn to control, all while dealing with the chaos of the Kekkai Sensen world.
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 idea could be a crossover with another popular anime like 'My Hero Academia'. The characters from Kekkai Sensen could enter the world of My Hero Academia and have to deal with the different power systems and challenges there. For example, Leo could try to understand how quirks work compared to his own abilities.
To write a compelling Leonardo in fanfiction, first focus on his existing traits. He's brave, so show him facing his fears head - on in new and unexpected situations. For example, if he has a fear of heights, put him in a situation where he has to climb a tall building to save someone. Also, explore his relationships. Maybe he has a falling - out with a close friend and has to work hard to mend it, which shows his loyalty and determination.
Maybe Chain is revealed to be working for an organization that has ulterior motives regarding Leo. However, as the story progresses, she realizes she doesn't want to go through with their plans and instead decides to protect Leo at all costs. This would be a great way to show her character development and also create a lot of drama in the story.
Sen Sen was a character in " Full Expert ". Sensen appeared many times in the novel and interacted with other characters. However, specific information about Sensen's background, personality, and storyline was not found in the search results provided. Therefore, based on the information provided, I am unable to provide a detailed answer regarding Sensen.
While waiting for the anime, you can also click on the link below to read the classic original work of " Full-time Expert "!