Honestly, scrolling through the Muichiro/Nezuko tag feels like a solid 80% of it is variations on two core ideas, and one of them drives me a little nuts. The most dominant one is definitely the 'Caretaker' theme, where Muichiro, after regaining his memories, becomes hyper-aware of and protective toward Nezuko. It makes sense—his regained empathy plus her enduring innocence creates a natural dynamic. You get tons of fics where he's teaching her how to communicate with the slate again, or just sitting with her during sleepless nights, or getting quietly furious on her behalf when other slayers underestimate her. It's sweet, often leaning into found family with the Kamaboko squad.
But the theme I see just as often, and that I find way more creatively limiting, is the 'Sun and Mist' soulmate or reincarnation AU. It’s always the same imagery: their past lives as lovers or siblings tied to Yoriichi and his family, their bloodline arts manifesting as literal sunbeams piercing through mist. It can be beautiful when done sparingly, but after the fiftieth fic where Muichiro has prophetic dreams about a girl with pink eyes who 'melts his foggy heart,' I start skimming. I wish writers would push past that symbolic shorthand and explore their dynamic in, say, a modern AU where a socially oblivious chess prodigy meets a selectively mute art student, or a scenario where they're forced to collaborate on a mission without Tanjiro as a buffer. The potential for quiet, understanding between two characters who've experienced profound loss and alteration is huge, but it often gets buried under overly familiar tropes.