Maybe Civ should add the capability to draft mercenary armies. These would have very low maintenance cost (say 25% of normal cost), but when you capture enemy cities you get a very small percentage of the loot (only 25%). Most would be kept by the mercenary forces. Also, as they moved over foreign land, if they can see an improvement, they will always waste a turn to pillage it for very little gain to you.
You would not be able to create an unlimited amount of them. In fact you would be required to maitain a force of equal strength or greater of regular units in the battle field or risk that they might rebel against you by forming their own empire from captured cities and pillaged wealth.
Mercenaries should require 2-3 times as much gold, but no resources as upkeep.
You would be in total command of them, but that's certainly true that they would loot for themselves.
Anyway their rebellion should happen two ways:
1) An enemy spy gives them X gold, and so they have a chance to change side to the spy's owner.
2) You go bankrupt, so as they realize that they can't get their money, they change side to the enemy... or become an independent AI civ, in war with yours. Try Rhye for the good implementation of Idependent civs.