I've heard the idea once and there's a certain lure. Maybe they should only appear if nobody did any ZC in certain parts of the map.
Lore-wise, it could be something like The Box in Van Buren's Denver - its essentially some central computer (not a AI) running off some riot control/enemy takeover protocol, and the way it sees is that Phoenix is overrun with rioters because it keeps coming short every time it tries to contact and find the proper lawful authorities - therefore Phoenix is essentially total anarchy zone (which is the right conclusion) because the computer cannot contact nor receive orders from the proper authorities because the city is now under control of communists/rioters/secessionists/something and it may having its communications blocked by hostiles (which is the wrong reasoning) - its too dumb to see that a nuclear war has happened and all its masters are centuries-long dead.
So it reacts by getting hidden robot factories active, cranking all sorts of robots from its hidden robotic assembly lines and doing everything to continue the protocol - like scavenging any necessary materials to build more robots, linking itself to once-inert power sources, sending robots to take the city and supress/anihilate anything that doesn't like Law & Order - which is pretty much
If you guys want I can show and describe the robots from FO1/2/FOT that could totes work in this situation.
I don't see this as having any place in the fallout world, just in regards to every single item of lore I've ever read. Could be a cool feature and whatnot, but I would be against it because it's just not Fallout.
Easily solvable, just make it khans or raiders, any of the badboys from fallout.
Enclave/Bos could fight over town as well.
Raiders might be a "lower level" version of this as well, with different tactics and weaponry.
Disagree on Enclave/BOS, they don't work as "mob" sort of enemies - the average Enclave trooper or BOS Paladin should be able to wipe entire groups of players, alone. Enclave is low-key during 2220s and if BOS wanted to take the city, they would simply send squads of Paladins into the four faction bases and kill everyone, the end.