How about free trades? If 2 civs really likes each others, they could sign a free trade agreement which make possible for both parts to access to strategic resources/luxuries of the covered zone!
For example, my neigbor Xerxes has some Iron to spare, and I, Hammurabi, have plenty of Oil and Ivory. With a free trade agreement, I could build railroads in my country, but Xerxes could also build Bombers and have a happier population.