The ending of Devils Line is intense. It brings closure to some storylines while leaving others open-ended, creating a bittersweet conclusion for fans.
You can try fanfiction websites like Archive of Our Own (AO3). It has a large collection of fanfics from various fandoms, and there's a good chance you'll find some great anzai and tsukasa fanfics there.
One popular trope could be the 'forbidden love' aspect. Since they might have some obstacles in the original story, this can be explored in fanfiction.
You can try looking on some fanfiction websites like Archive of Our Own (AO3) or FanFiction.net. However, make sure to follow the rules and guidelines of those platforms.
There's also a fanfic called 'Tsukasa and Anzai: The Bond'. This one focuses on how their bond deepens over time. It includes some really heart - warming scenes where they support each other through difficult times. It's a great read for fans of their relationship.