Personal Finance, Trading and FIRE are three under-represented themes thus far

HumansBeingBros

Some of the more niche ones, such as music composition or Euroleague Basketball. I doubt enough people migrate to a actually create a somewhat active community for those.

Baby/toddler parenting groups. I joined groups for the month/year of each of my little ones on Reddit and they're very active communities.

r/UnresolvedMysteries. I really liked this subreddit. Now I took a look at the thread about API changes which is stickied there and it just seems a lot of people are asking "can someone explain to me what's going on/who cares about 3rd party apps" (and my favorite one - "it's all power-hungry mods' fault"), so it doesn't seem like the community will migrate from reddit.

I miss reading all the write-ups, not only about true crime but also other mysteries.

I'll also add: LiminalSpace and ididnthaveeggs (for all the insane recipe reviews lol).

r/beermoneyuk for me.. it was a good place for me to find some additional sources of online income (really helped me out this year as a student). I've created the magazine @beermoneyuk, just in case (if the mods ever show up I'll gladly give ownership to them), but I'm not sure if enough people will visit it.