What you wish had an active community here on the lemmyverse?

Basically all the media.

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.