Along those lines, what if communities could suggest other related communities, and clients could (by default, but optionally) also show posts that the communities your subscribe to recommend?
It's a little like soft federation, but on the client side - or, think of it like an automatic way to make multi subs while still allowing users to change it, and giving mods a less nuclear option if they don't like how a community on another instance is going.
Hmm, 2.8 million subreddits, but how many are ghost towns? I wonder if anyone has a measurement in terms of monthly active users or something along those lines.