Actually I'm considering your earlier idea for game 4... Let it cost 10 resources to conquer a bordering zone, and if you want to move further in that direction, maybe 20 or 25, and then 30, 40 or something else...
With big cities everywhere, and especially in naval wars, there will probably be fronts which won't move at all, because we can produce much more than we can spend at attacking neighboring zones. The artillery bases will change that as the game progresses, ofc.