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.
Yosano could have a rather detached view. She might see the fanfiction as a form of escapism for the fans. However, if the fanfiction involves Dazai in some dangerous or life - threatening situations, she might be a bit more invested, thinking about how she could've saved him in those fictional scenarios.
Well, some characters might be amused. For example, Atsushi could be a bit shocked at first, seeing the different interpretations of Dazai in the fanfiction. He might find some of the more out - there scenarios quite strange but also interesting in a way.
One great 'bungou stray dogs dazai x chuuya fanfiction' is 'Redemption in the Rift'. It beautifully explores the complex relationship between Dazai and Chuuya. The author does an amazing job of portraying their characters' inner thoughts and the push - pull dynamic they have. Another good one is 'The Bond Beyond Ability'. It shows how their relationship can develop in unexpected ways outside of the main story's battles and politics.
One common theme is mentorship. Dazai often acts as a mentor to Atsushi, guiding him in the use of his ability and in navigating the complex world they live in. Another theme is friendship. Despite their differences, they form a strong bond of friendship that is often explored in these fanfictions.
AO3 is probably the best place. It has a great tagging system, so you can easily search for Dazai x Chuuya fanfictions. There are also some Tumblr blogs dedicated to sharing fanfictions in this fandom. You might need to do some digging, but you can find some real gems there as well. Some Discord servers related to the 'Bungou Stray Dogs' fandom also share fan - created works including these types of fanfictions.
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.