Well, a lot of fanfics also pair Atsushi with different characters in a romantic way. For example, there are some that focus on an Atsushi x Kyouka relationship, showing how they support and grow with each other. These types of fanfics can be really sweet and add a whole new dimension to the characters' relationships within the Bungou Stray Dogs universe.
One great fanfic could be 'Atsushi's Feline Adventures'. It delves into Atsushi's experiences as a cat in the world of Bungou Stray Dogs. The author beautifully portrays how he uses his cat - like instincts while still having his human - like intelligence. It's a really interesting read that shows a different side of Atsushi.
One common theme is his self - discovery. Atsushi has a lot of insecurities and a troubled past, so many fanfics explore how he comes to terms with who he is. Another theme is friendship. His relationships with the other members of the Armed Detective Agency are often central, like how Dazai's mentorship affects him.
I also like 'The Unseen Connection of Atsushi and Dazai'. This fanfiction takes a different approach. It starts with a mission where they get separated and then shows how they both realize how much they rely on each other. It has a great balance between action and the development of their relationship.
Archive of Our Own (AO3) is a great place to find such fanfictions. It has a large and diverse collection of works, with various tags and filters to help you narrow down your search to exactly what you're interested in regarding Atsushi and Akutagawa.
The theme of redemption can be found in many of these fanfictions. Dazai, with his past sins and his current state of mind, might be seen as on a path of redemption, and Atsushi can play a part in that. Atsushi's pure - hearted nature can inspire Dazai to be a better person. Additionally, the theme of trust is important. Given Dazai's unpredictable nature, Atsushi has to learn to trust him, and Dazai has to decide whether he can trust Atsushi as well.
In bungou stray dogs fanfiction, Dazai and Atsushi often have a complex relationship. Dazai is like a mentor to Atsushi in some ways. He guides Atsushi in the world of the Armed Detective Agency, teaching him about his abilities and how to survive in this dangerous world of gifted individuals. Their relationship also has elements of friendship, with Dazai's often - eccentric behavior being a source of both confusion and learning for Atsushi.
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.
The scenes where Atsushi interacts with the other characters as a cat can be really great. In 'The Cat Amongst the Stray Dogs', there are these really cute moments when he curls up next to Dazai and Dazai just pets him while they have a deep conversation. It shows a different dynamic between the characters.
One possible plotline could be that Atsushi's sickness is a result of overusing his ability. His comrades in the Armed Detective Agency would then rally around him. Dazai might try his comical yet somewhat effective ways to cheer him up. Kunikida would be strict about his recovery regime, making sure he takes his meds on time and gets enough rest.