One common theme in Atsushi x Kyouka fanfiction is the idea of them facing challenges together. Whether it's dealing with new threats to Yokohama or internal struggles within their respective organizations. Their relationship can be a source of strength for both of them. Some fanfictions may also delve into a potential romantic aspect, though it's not always the main focus. It could be a slow - burn relationship where they first develop a deep friendship and then, over time, something more might bloom.
One popular trope is the 'protective Atsushi'. Since Kyouka has a troubled past, Atsushi is often portrayed as being very protective of her. He doesn't want her to be dragged back into the darkness she came from. Another trope is 'growth through each other'. They both have things to learn, and fanfiction writers like to show how they help each other evolve as characters.
Well, you could say there are hints of love here and there. But it's not a conventional love story. The series is primarily known for its action and supernatural elements with love being a secondary aspect.
Bungo Stray Dogs is both. It began as a light novel and then was transformed into a manga. The light novel provides detailed storytelling, while the manga brings the characters and scenes to life visually.