So, I think Communities is supposed to be something like what you say, because they are definitely supposed to play off of the current tag structure. There will be moderators and administrators which can make up their own rules. What I don't know is if the current tag system will be separate, or if it will be up to each Community to keep things free or not.

I don't know when it's supposed to happen, and I don't know if it will be exactly what you're talking about, but it's been in the works for a while and it's supposed to be "around the corner." :)