I feel the very specific community topic split is already affecting Lemmy negatively. So I think having larger, broader community topics (e.g. 'commuting' instead of a community for every single option to commute by itself), with more diverse content, interaction and of course more visible activity, would also attract new users.

Right now some communities are so specific, that by its creation, it's a filter bubble by design. And then of course you don't get a lot of content or interaction, as only yea-sayer get accepted.

Interaction requires different approaches, opinions, options and of course people who upvote them even when disagreeing. The reply box is the correct option when disagreeing, not the downvote. That's how Lemmy will sprout.

tl;dr Broader community topics for larger, more diverse and more active communities