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

r/mbti 16 mbtis

r/infp

r/ultralight, r/myog and r/wildernessbackpacking

You can tell what I'm into.

I really enjoy r/archery hope they make it here

Hobbydrama It was kinda interesting hearing about the inner workings and happenings in hobby's you never even heard about.

A couple, I think. WoTshow is one I really liked. And exatheist is a tiny sub where a bunch of people who were once atheists of all different religions talk religious philosophy.

Just the really small ones i was subscribed to, Tasker will probably be my most missed one, hard to find another forum specifically for one app!

r/kamenrider, r/supersentai or any of the tokusatsu subs for that matter

r/piracy

r/electronics

r/rce

r/diy

r/diyaudio

r/diwhy

r/edm

r/edmproduction

r/angryupvote

r/masterhacker

r/mathmemes

r/sciencememes

r/physicsmemes

r/chemsitrymemes

r/bilogymemes

r/onejob

r/outoftheloop

r/printers

r/redneckengineering

r/torrents

r/tvtoohigh

r/winehq

r/youtubedl

r/weird

Conlangs!

This seems way too unlikely to happen, but r/askhistorians. It's an exceptionally well moderated (if a bit overzealous) community that allows for both the average person to ask seemingly impossible, very specific questions as well as academics to express their knowledge and passion for those very same super specific topics they find fascinating; but have difficulty finding an audience to listen to them in their day to day lives.

Waiting on r/linuxmasterrace tbh

r/HobbyDrama or a more hobby oriented history community. Also r/AskHistorians or similar, although that one will take more effort and setting up than simply making the community. I personally use Reddit to see a couple memes and then read both of the subs I mentioned. I like seeing people who are passionate about a thing talk about the history of that thing.

r/ubiquiti

some of the more or less interesting ones i came across...

r/InternetIsBeautiful r/AlpineLinux r/CoOpGaming r/antiXLinux r/devuan r/gamedeals r/privacyhardware r/opensourcegames r/FreeGaming r/fossdroid r/linux4noobs r/commandline r/channelsofyoutube r/DeepIntoYouTube r/ChannelWatch r/EatCheapAndHealthy r/Foodforthought r/initFreedom r/irc r/KeepOurNetFree r/linux_devices r/suckless r/techsupport r/unknownvideos r/usenet r/youtubeclassics r/suggestmeabook r/thinkpad r/DistroTube r/odysee r/lbry r/peertube r/FOSSPhotography r/DepthHub r/TrueFilm r/socialscience r/AskSocialScience r/UpliftingNews r/LinuxPhones r/startpages r/terminal_porn r/nodrm r/Antifeatures r/FreeAudio r/listentothis (separated by genre)

A VR (virtual reality) focused community would be nice, it has a very strong community across devices all things considered.

I'll definitely miss r/framework the most, the community on the subreddit is awesome

I'm in a few different subs on Reddit, but the ones id love to see the most here would probably be:

r/stellaris r/crusaderkings r/kotor r/fireemblem and r/talesfromtechsupport

r/usbchardware

r/goldenretrievers

r/singapore

r/nottheonion

r/antiwork

r/impracticaljokers

r/outside. That place is such a needed mental break.

All of the Warhammer related subreddits!

r/degoogle r/linuxmasterrace r/linuxfornoobs r/Linux r/whatplantisthis r/eattheweeds

r/dmacademy

CastIron was one of the more niche subs in my list. Lots of good information for taking care of cast iron pans there.

r/oneorangebraincell, r/standardissuecat, r/GuiltyGear, r/CuratedTumblr