Specifically, instead of letting people join multiple villages, why couldn't there be a system implemented where villages give each other some kind of 'token' or 'item' (or alternatively, the chieftains come together) and form a Kingdom or Alliance. This would then allow more progression as villages could progress together. Also, this means that each chieftain would be able to give trespassing/stealing rights to the members of other villages in their Kingdom/Alliance (through a menu whereby you would be able to see each village's members and give them colours much like you can with Claims currently) and people working together currently could be more organised in certain aspects of war and peace (with maybe an Alliance chat as well as a Village one... Although the chat area is currently quite full what with Combat, Party, Village, Area Chat and Crafting already existing there so that's something to consider).
This isn't really a finalised idea so much as a starting point for a possible expansion upon the current endgame... You also may decide to create a diplomacy window whereby you could have a Kingdom of cities and also at the same time be able to initiate relationships with other Kingdoms; the difference being that you could only join villages into a kingdom if they are in a certain proximity to you (in order to stop a Kingdom from becoming fragmented settlements which span multiple supergrids or something) but also allowing for relationships to occur with friends from different Kingdoms at the same time (maybe with the learning of an optional skill, Emissary or something to that effect?)
I don't know what you guys think to this but it sometimes feels at the moment that despite having kin from other villages and such that you are isolated from the outside world... Ideas/suggestions/criticisms anyone?

Fredulom