I was going to suggest, what with the tribes and all, some inspiration could be taken from Fo:NV, but it appears BBeast had a similar idea, without having to throwback to some 2010 game.

Perhaps the Major City has access to some form of greater magic, either via some deity, powerful totem, or some major plot-twisty thing. *coughcouigh*Magical overlord pixies hellbent on enslaving the world*cough* 

As far as GMs go, I would have to vote for any combination of BBeast, Lugu, and Cyclone. You all seem to know what you're doing.