I'll just be honest, from my perspective on lemmy everything outside of porn, linux and shitposts is lacking. Interaction outside the top of hot is a wasteland of non-existence, questions go undiscovered, comments are never read. We could all be more generous with upvotes to improve visibility.
For me sfw art communities, sports, and life protips would all be nice to see grow. I miss the old photoshopbattles too, but I think that's just fallen out of style in general.
There is (or at least was) a special kind of joy in discovering a new piece of media (movie, TV, book, video game, comic, etc), getting to the end, and hopping over to the relevant subreddit to sort by "top of all time." Bonus points if you loved the series and would get to essentially relive it all over again through the sub, but even media that you hated or were neutral about could be fun subs to peruse; maybe you would get to revel in seeing something you hated turned into a meme highlighting how stupid it was, or get to feel justified in your negative assessment upon reading an epic rant from another user; maybe instead you'd find hidden details or explanations pointed out by other users that made you reassess the work ("huh, I though that was a stupid plothole but it actually was perfectly explained by that one scene that apparently went over my head"). The ATLA subs especially were treasure troves of tiny details and "holy shit I just noticed on my fifth rewatch" posts that really elevated my opinion (and thereby enjoyment) of a series I was initially kind of "meh" on.
When I think about what it would take to feel like Lemmy had sufficiently replaced Reddit for me, the number one practical answer is for comprehensive news (political, world, cultural, meme, etc... Reddit really did at one point feel like "the front page of the Internet" if there ever was one), and the second is to have the critical mass to be able to ask a question and get a good recommendation for any specific product or service, via regional subs, hobby subs, etc (although thanks to LLMs and corporate astroturfing that may simply be a bygone part of the Internet). But the "fun" answer is to have the critical mass for a wide range of specific fandoms.
A lab work group, like that one on Reddit. I cannot remember the name and I sure as hell will not go to that damned site, but it was basically full of graduate students and technicians that shared stories from their labs.
There are knitting/crochet communities of course, but all the super niche ones like ply-split braiding or smocking are too rare to warrant a whole community to themselves. On reddit there was a defunct sub called bistitchual, both for all obscure fibercrafts and for combinations of unrelated fibercrafts in one work. I wish we had it here.
More outdoors stuff. They exist, but not very active. Mountaineering, climbing, camping, overlanding, etc. Love people sharing their adventures and all the gear and tips & tricks discussions.
Also, I'm really confused as to why chess doesn't have an active community on Lemmy. Online chess has seen a big boom in recent years, and the demographics of chess players and Lemmy users should have a lot of overlap (i.e. the nerdy IT people), but for some reason the chess community here is more or less dead. Only anarchychess is active, which is great, but I'd love to have an active replacement for r/chess.
For me, it's any community of Tradespeople. I can find relevant manufacturer and adjacent code regulations for modern equipment or building techniques anywhere online. The problem comes from obscure-ancient technology that was discontinued 60+ years ago, the only references to those are on Reddit and very specific forums.
I recently ran into an electrical panel that was built in the 60's and was promptly made illegal (split bus residential panel, no singular main disconnect switch). Even being trained and educated as an Electrical Engineer, it only gave me the ability to understand what the panel was doing, not the history and use cases of the past (since their use in residential applications is obsolete). I was able to find discussions between inspectors and electricians, how things played out with local authorities, and the on going debate of their practicality by actual professors discussing regulations and safety. I will miss these resources if they become unavailable at a future date (the whole enshitification process).
That being said, places with higher than average traffic (like reddit now) tend to give a lot of crappy answers. Lot's of diy'ers thinking their way is best (whether it's code compliant or not), and others who don't care about discussion and only want to say you're doing it wrong because it's not how they would do it (and nets them the highest profit margin on a job). There's lots of owners out there that are probably afraid to ask a question now adays because of the responses (same linux community effect), even though the information around it could be important.
Terrariums. I love miniature things, including tiny ecosystems. There’s a few communities on Lemmy, but they’re mostly inactive, and have a tiny amount of subscribers.
Reddit had quite a few, pretty popular Buddhist subs. There isn't even ONE buddhist sub here with more than 3 active ppl. And those are usually the same person posting. I still use reddit from time to time in my phone browser just to check them out, but maybe one day we'll have more on lemmy.
Would love it if there is actually an active Tea community on Lemmy. The ones on Lemmy are pretty much dead. The Reddit one is super active.
Also, Lemmy doesn't have active communities for individual games like Reddit does. They are useful to obtain information, get help, talk about builds, etc
A Canadian politics and current affairs community.
A community for some of the podcasts I listen to would be nice.
We have a few of those here, but they aren't too active and they have a pretty narrow Overton window (ie, I tend to agree with most posts and comments).
I tried posting that kind of stuff. It was thankless. Nobody else started posting. I gave up.
Religion. I love talking to people about what they believe and why, and other philosophical discussions, but the two communities I created for those types of discussions have been dead since the Reddit exodus.
I'd love to see more healthy food, fitness, weight loss and healthy lifestyle-related communities! Also, I don't think there's a community for 'petite fitness'? Would love to see that as well.
Oh I guess "an active community for fanfiction of this specific TV show or videogame I like to enjoy" would be far too niche, right?
Fine, then I'll say immersive teaching (using dioramas, doing experiments on the field, etc... for teaching classes), and alone / 2-people living lifehacks (in particular in this economy).
Seeing as this thread is still active, instead of continuing to reply to people throughout, gonna go ahead and put this out here.
If you're not finding an active community for something (safe for work, that is) or any community whatsoever for your interest, you're welcome to post about the topics that interest you in !general@lemmy.world till you find enough likeminded people to get a separate community going. This was always allowed tbh, but I've tried to make it more explicit and clear that it's cool.
Nim-lang (this has some activity here, though I often have federation issues)
Raylib
low-poly+untextured polygonal art and vertex colors (both 2D and 3D)
I don't really consider myself an artist or a programmer (I haven't done much), maybe there's a fediverse instance that could work for me but it's probably too niche even just with those communities.
A Tolkien based active community would be great. Reddit had/has a few that were fairly active. There are communities here but really not active at all.
Also wish the NBA community was more active - quite frankly surprised it's not.
On reddit, there's a subreddit called r/lrcast, which is the dedicated subreddit for the Limited Resources podcast. The primary purpose of the subreddit, however, is not to discuss the podcast, but to discuss the "limited" format of Magic: the Gathering, which constitutes draft and sealed. It's a very difficult, very expensive format of Magic to play and is a niche subsection of an already fairly niche hobby.