The tree is just me getting carried away with elaboration. The basic idea is you have one community higher than the first one. The lower one can be completely o

The tree is just me getting carried away with elaboration. The basic idea is you have one community higher than the first one. The lower one can be completely open; the higher one can be invite-only. (Oh, there's that essence I wanted.) Maybe the two levels are a little like a freemium model, where the analogue of free vs. paid is anyone vs. invite-only. One is a recruitment centre for the other. Perhaps the same submissions and comments would be presented to both but rated by their own communities; or perhaps it would be like subreddits, totally distinct in their stories and discussions. The difference from subreddits is it's not flat and not all-welcome, but hierarchical (in both a logical and political sense). Maybe SO does this a little with karma-based privileges, but there isn't a higher-level forum (at least, not one that I'm aware of...)