For a long time I've wanted to play/create a server like this.
The idea is to make it as vanilla as possible, with maybe 2 or 3 set kingdoms that players can join, build, be a part of.
The kingdoms would ideally be as community-ran as possible, with each kingdom free to make their own decisions such as government style, leaders, taxes, diplomacy with other kingdoms.
If this is something you think you would be interested in, let me know. If I get enough responses Ill go ahead and create the server.