Congratulations! It is one of the most addictive games than I have played lately, and in my opinion part of the big success is due to his easy and intuitive control (between a lot of other factors!) But go to the topic: The Slavery. (Excuse my poor english, hope you can understand what I want to explain.
In the Ancient Age, the slaves were the main manpower in most of cultures and empires: Persians, Egyptians, Greeks and Romans... So the use that of them does the game is correct: Manpower in the mines and armies food supplies transport (though the slowness displacement of these and the sheep flocks complicate sometimes the speed of development of some campaigns.) I believe however, that an additional use you could give them would settle one of the most usual complaints of this forum : The lack of foods.
My suggestion consists of stack up a unit of slaves (40 men) in a city. As the military garrison units provide bigger defence level, a slaves' unit could increase the speed of each and every one of the terrestrial supply lines (+15/wk) This way, the speed of the most distant route between two cities that had a slave unit working on each one of them would be of: 15/ wk (basic) + 15/ wk (slaves unit in city A) + 15/ wk (slaves unit in city B) = 45/ wk. I believe that this improvement would be enough to guarantee a fast supply once the route was set, much between cities as to farms and distant mines, eliminating the micromanagement of the units of workers and slaves. The stacking up only would have effect on the cities (no effect on mines and farms.) Besides, you could limit the number of slaves by city: A 40 slaves' unit for cities with lower population capacity than 12 and two units (80 slaves) maximum for cities and capital cities of 12 or more.
The commercial routes also would increase his retribution in +1g by each piled unit of 40 slaves in the city, as compensation for destine slaves to different tasks to the gold extraction in the mines. The improvement can be justified even from the historical viewpoint: To the two already commented uses, the slaves in the cities were used for the transport and storage, they built and improved the paths and communication roads (accelerating, therefore, the transport and distribution of commodities) besides being source of income straight (slaves' market) and indirect (released from the basic tasks, the citizens improved the Arts and the Sciences.) The speed improvement and the bonus can apply also to the maritime routes: Goods loading and unloading also is more efficient.
This is my suggestion. Although it doesn't solve the lack of food totally, combining an adequate planning of the campaign according to the year season and the routes between cities, would be possible to move the food to the front cities with enough more speed than now . Besides, It would give extra value to the pursuit and conversion of routed units to enslave them. Well, that's all. Greetings from the Mediterranean and one more time big thanks for a great game.