What are some communities that aren't here yet that you hope to see in the future?
For me it's been communities like /r/buildapc, /r/buildapcforme, /r/buildapcsales, /r/gamedeals, and /r/consoledeals have been useful throughout the years.
r/tipofmytongue (iirc that name - the one for helping people find shit they've forgotten)
maybe r/52books (it produces too many simple posts to just go in r/books, not sure if the same would be true here)
r/vegetarian
Edit: r/fantasy is the biggest one for me. In effect the subreddit is really more for SciFi and Fantasy, both - and other speculative fiction - and they have a yearly speculative fiction bingo card that's really fun to participate in. It's mostly books in practice but is not exclusively about books.