Mastodon lets you subscribe to hashtags. Misskey/Calckey let's you create saved searches for termsaand hashtags.

Community tags and either of those options would go a long, long way.

Both also have lists. Being able to add communities to lists would give people the "metacommunities" they think they want.

But honestly, I think people will do better long term if they have to put in even just a little bit of legwork to find the communities with the right fit, and ignore the rest. People have a lot of FOMO around this, but it's not like anyone read even 1% of anything that was ever posted to big subreddit. They never feared missing out on all of the stuff below the fold.