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.
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.
You can start by looking on fanfiction websites like Archive of Our Own (AO3). It has a large collection of fanfics from various fandoms, including ones for kyouka x atsushi. Just use the search function and type in the relevant tags.
One common theme could be the rescue. Other characters might band together to save atsushi. Another theme could be atsushi's inner strength while being kidnapped, like how he copes with the situation mentally.
The fanfiction 'In Dazai's Shadow' is also worth checking out. In this story, the OC is a shy artist who gradually opens up to Dazai. The story is not only about their relationship but also about the OC's personal growth and how Dazai influences it. The writing style is very descriptive, making it easy to picture the scenes.
I'm not sure about specific 'atsushi kidnapped' fanfiction directly. But you can try searching on popular fanfiction websites like Archive of Our Own. Just use the keywords 'atsushi kidnapped' in their search bar, and you might find some relevant works.
First, you need to have a clear idea of your OC. Decide on their personality, backstory, and what makes them unique. For example, if they're a strong - willed journalist, that will shape their interactions with Dazai. Then, think about the setting, like a modern - day Yokohama or a fictional alternate universe.