Now, to start off, I want to say I actually approve of the current trade model, where each colony, outpost, and market contributes to trade capacity, you get a fraction of trading partners' trade capacity added to yours by trade agreements, and the increased value of foreign trade means you'll see an increase in income from opening trade even before building more freighters to handle increased capacity. What unnerves me is the lack of sanity checks to make sure the 'foreign trade bonus' doesn't absolutely swamp the size of the domestic economy- something I see frequently with my trade partners in my own games as a trade- and Market-happy Phidi player.
I would think that it doesn't really matter how huge a market a neighbor provides, after a certain point you'll saturate a given society's ability to take advantage of export and import opportunities. The trade value reaped by a hypothetical single independent town opening trade with the United States is unlikely to be significantly greater than it opening trade with, say, Guatemala. Any thoughts by other players, or by the devs?