One interesting Neuvillette fanfic could be 'Neuvillette's Journey of Discovery'. It might explore his character in-depth, his thoughts and how he copes with the various situations in the world he inhabits. Another could be 'Neuvillette and the Mysterious Case' which could involve him solving some sort of enigma in Fontaine.
In atsushi time travel fanfiction, Ranpo can be a great character to pair with Atsushi. Ranpo's super - detective abilities can play an important role when they are in different time periods. Chuuya is also a popular choice. His powerful ability and his connection to the Port Mafia can add a lot of drama to the story. And Yosano is another one. Her ability to heal and her strong personality can make for some interesting interactions with Atsushi during time travel.
You can start by looking on Archive of Our Own (AO3). It has a large collection of fanfics from various fandoms, including those about Neuvillette. Many talented writers post their works there.
Most BSD light novels often have complex characters. Since they are based on literary figures, they bring in the depth and complexity of those original personas. For example, a character might have the same traits as a famous writer but with a unique twist for the BSD universe.
The appeal of all BSD novels lies in their ability to create immersive worlds. They draw readers in with their unique concepts and engaging storylines. For instance, the way they handle character relationships is very captivating. The conflicts and reconciliations between characters make for a great read.