I have a suggestion to add gold as a resource which can only be used to train mercenaries from neutral buildings and can be used for market exchanges. And it is gathered by unique way. Luxury resources like coal which are randomly shattered on maps can only gathered by merchant and can only be dropped at building called merchants tent. When the storage capacity of tent is full then merchant sets out to exchange that unit of coal with gold from a neutral trading post like building which is situated far away from the town maybe at corner of map. The trading post can be used by any civ. When merchant reaches trading post he returns to market and drops gold there. On the way other opponents can attack merchant and steal goods so the merchant can be guarded. Player can also build trading camps(which also act as mini defense tower) to ensure the route for merchant and make him have player-willing path. I know adding a resource isn't that easy. You people have to balance the gameplay too. If you like it use, modify it.