Basically, if trade is to move through territory or waters controlled by another civ, you would have to enter into a trade agreement with them first. This could be done through the trade screen that we use now to set up open borders. So you could either open trade with another civ for free or charge them something for it. The other civs would have to do the same. The system would "flavor" according to other civs opinion of you, making it very likely for easy "free trade agreements" in the early game, but get harder as the game progressed. Would a system like this be possible? My knowledge of modding is non existent. I like to control waterways and sometimes place cities to block one off. Just burns me when I see a trade ship burn through one to reach some other civ. No benefit to me.