Not a change I would want in the game but as a mod.
It would be interesting to have real cheap units, Say 10% of the cost they are now but with a continuous cost. Dont want to put a number on it but lets say X% of cost per minute. It would reflect more a real life cost in maintaining an army.
Further instead of deleting a unit and giving experience to an opponent, a unit could be disbanded, making the unit disappear so you dont have to pay the ongoing cost (BIG BRAIN - Pay out the unit to become a villager or pay a villager to become a military unit).
If you dont have the resources to pay for the units ongoing costs then they could have a chance to autodisband or go rouge, becoming an aggressive Gaia unit or worse! become an enemy unit.
I know this would completely change how AOE3 would be played and potentially makes it into a different game all together. This is more a thought experiment at this point then something that should actually made.