I just read an interesting isekai, harem uppermoons x reader (gender neutral) fanfic on ao3 called the awakening of the demon king by wholesome_sliced_bread and plot so far is amazing!
In a nutshell, reader found a dead Muzan and ate him and they became the Demon King. (I find this terribly disturbing and funny at the same time).
You were just a student but then you stumbled across Muzan’s corpse and nom nom nom. ✨ expensive mukbang ✨ You got isekaied to the Demon Slayer world. (I drew another version of reader for another fanfic— I’m not running out of ideas at this point. But I drew them female huhu I should’ve drawn them ambiguous.)
Since you became the Demon King, all of the demons under Muzan’s creation, especially the Twelve Kizuki are under your control.
The original story of Demon Slayer might change depending on Reader, who is a fan of Demon Slayer in the real world, meaning they know everything what will happen to the story, who dies and survives, and how the original story will end. The fate lies upon their hands.
I enjoyed the story so far. I recommend everyone to read it~