The first part of what you say is similar to what's happening with some parts of Mastodon. There are many, many small instances that are (apparently) starting to develop their own cultures, but you can still see what other people are saying because of federation.

The ideas of requiring engagement, and mixing the groups, are interesting, but I'm not sure how they'd work. I've seen the "You must engage" idea applied elsewhere without success, so it needs a little more work.

Even so, I think your ideas have promise.