!cyberpunk - mostly games, movies, shows, and music in the cyberpunk sci-fi genre
!wavemusic - music: synthwave, vaporwave, etc. A fairly new community, a couple different people have been posting
!gothindustrial - music community. I was hoping someone else would make the 300th post, if not I'll post something later today.
!fullmoviesonyoutube - a larger community, a couple of us are posting regularly, but could use more discussion
!fedigrow - a community about how to help grow the fediverse. @Blaze has been posting a weekly thread on "how is your [niche] community doing?" which is kind of like a support group for people keeping communities alive
edit: how could I forget, !shortstories - links to short stories online in all genres