I've spent way too much time scrolling through fics for this series, and honestly, the themes feel incredibly specific to me. A lot of stories fixate on Shiki's inner conflict and guilt, that heavy weight of responsibility he carries. You get tons of fics exploring his relationship with Akiha, but it's less about sibling fluff and more about the toxic, codependent pull between them. They're two broken people holding each other together while also tearing each other apart, and writers really dig into that twisted dynamic.
Beyond that, a huge chunk of stories are about atonement. Shiki killed someone important to him, and the fandom is obsessed with him walking that path—sometimes literally, through bloody battles, sometimes through quiet, painful daily life. The 'beast of justice' ideal gets twisted too; is he really saving people, or just feeding his own need for punishment? I've seen fewer comedy or slice-of-life fics, which makes sense. The source material's tone is just so grim, it sets the mood for most fan creations. You really have to be in the right headspace to dive into this tag.