A lot of these aren't exactly "niche", but afaik none of them exist on lemmy yet

Edit: for some reason, a bunch of these that actually do exist weren't showing up in search unless I specifically searched for "!community@instance". I've gone back through and added links to them here.

r/actuallesbians
~~r/anarchychess~~ !anarchychess@sopuli.xyz
r/anime_irl
~~r/animemes~~ !animemes@lemmy.ml
r/beyblade
r/bropill
r/conlangs
~~r/datahoarder~~ !datahoarder@lemmy.ml
r/ddlc
~~r/egg_irl~~ !egg_irl@lemmy.ml
r/femboymemes
~~r/forbiddensnacks~~ !forbiddensnacks@lemmy.ml
r/gravityfalls
r/heraldry
r/heraldrycirclejerk
r/infinitytrain
r/katawashoujo
r/leopardsatemyface
r/logh
r/lycorisrecoil
r/madeinabyss
r/madokamagica
~~r/math~~ !math@lemmy.ml
r/mathmemes
r/mbtimemes
~~r/mechanicalkeyboards~~ !mechanicalkeyboards@lemmy.ml
r/mushokutensei
r/musictheory
r/nails
r/nextfuckinglevel
~~r/oshinoko~~ !oshi_no_ko@lemmy.ml
~~r/philosophy~~ !philosophy@lemmy.ml !philosophy@mander.xyz
r/photoshopbattles
r/playstupidgames
r/pokemongo
~~r/portal~~ !portal@lemmy.ml
r/princessesofpower
~~r/programmerhumor~~ !programmerhumor@lemmy.ml
r/shitduolingosays
r/shitpostcrusaders
r/stardustcrusaders
r/stevenuniverse
~~r/swordartonline~~ !swordartonline@lemmy.ml
~~r/talesfromtechsupport~~ !talesfromtechsupport@compuverse.uk
r/the10thdentist
~~r/thinkpad~~ !thinkpad@lemmy.ml
~~r/tokipona~~ !toki_pona@lemmy.ml
r/touchfluffytail
~~r/traaaaaaannnnnnnnnns~~ !traaaaaaannnnnnnnnns@lemmy.ca
r/transadorable
r/transprogrammer
~~r/transhumanism~~ !transhumanism@lemmy.ml
r/transtrans
~~r/visualnovels~~ !visualnovel@lemmy.ml !visualnovels@lemmy.comfysnug.space
r/wholesomeyuri
r/yurimemes