In order to liven the game it makes sense to introduce more options for trade. The current trading system is one of the weakest links in the Galactic Civilization series: send or receive a ship for trade, and you receive money. There is no trade-off (pun intended) to this exchange, nothing to make it more interesting than gaining credits. In order to make it interesting, trade should be modified in the following ways.
For starters, trade should be possible both within and without an empire. Foreign trade will be encouraged over internal trade by the possibility of access to wealthier and more advanced markets than domestic markets, but will ultimately limited by a lack of trade licenses. Domestic trade will be unlimited to the extent that the player can construct, pay maintenance on, and protect trade ships within one's borders, as well as to the extent that the player desires and needs trade.
There should be two kinds of trade: food trade and consumer goods trade. Food trade will consist of sending a portion of the agricultural production of one planet to the other, while consumer-goods trade converts a portion of total social production on the planet into consumer goods that are sold to the other planet (much like Economic Stimulus). Based on the existing GROSS income, population, and food production (for food exports) of the planet, a price distribution will be created, using a log-normal distribution to define which per unit price would result in the maximum profit, the default cost of production for a unit of food or consumer product being 1 BC.
The revenue generated from trade will be added to the gross income of the sending planet and subtracted from the gross income of the receiving planet. Through this model of trade it will be possible to account for the wide variety of markets present throughout a game. Additional food on a planet will allow additional population to exist there, while exported consumer goods will increase approval (as should Economic Stimulus, to a lesser extent).
Trade, however, would not be solely a zero-sum game because value is added economically on both sides of the transaction with warehousing, distribution, packaging, retailing, etc. These should be set at 5% of price given on each side for domestic food transactions (7.5% foreign) and 10% on each side for domestic consumer goods transactions (15% foreign). In the case of two-way transactions, the effect would be doubled since the food and consumer goods are considered to be unique to each planet, but further recirculation (sending goods from planet A, to planet B, to planet A, to planet C) would result in a compounding increase in price required to make a profit based on the appropriate value added. In other words, the 1 BC cost basis mentioned earlier would rise, leading to lower sales and profitability. Trans-shipments would see a rise of 2% on food (4% foreign) and 5% on consumer goods (10% foreign), and could thus be made viable by the consolidation effect of having less freighter-distance, etc.