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.
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.