A well known feature from Reddit, default communities (subs on Reddit) are communities that newcomers are subscribed to by default. Lemmy, and specifically lemmy.world, could use some of these, I feel. At the very least, communities like lemmyworld, general, and newcomers are good ones to include, if we're still somehow sticking with the old Lemmy ethos of less guided interaction. Aww, pics, videos, memes, news, etc, are good ones if not. This massively sped up the integration of new users on Reddit, and I believe it's a good addition to Lemmy.

Added on to this is a capability that Reddit had and lemmy doesn't yet, which is multi(reddits) communities, or Collections is probably what we'd call them here. I could see a 'default' collection being applied to new users, for example. The pie in the sky version of this would be publicly browsable and shareable collections, so you could send your friends a link which allows them to subscribe to multiple communities at once and create a new personal Collection automatically based on it.