I don’t want random posts to appear in my feed from communities I haven’t subscribed to, but I want to have a feature that shows me suggestions for communities when I ask for it. That’s a big difference. Right now it’s (too) hard to find these communities.

Yes, that’s what I mean by not trivial, a centralized system can do analysis like this a lot easier. But even on your own instance, they could find the N users with the most overlapping subscriptions and check which communities they follow to give you recommendations.

I would like to see recommendations for communities based on my communities. It’s not trivial to solve, but discoverability isn’t great right now.