I don't know about you guys but I have never been very satisfied with the trade mechanisms in 4x games. The best system I've seen would have to be Civilization, but that suffered from the way its resources worked (you either have a resource or not, quantity is irrelevant). Another flaw is that everything was instantaneous; trade would happen immediately, even if it would take my fastest units 10 turns to reach the trading partner.
Actually now that I think about it, Colonization does trade better. Its resources work closer to how they will in EWoM (i.e. quantity matters), and in order to trade you had to actually send a caravan or boat to the destination. What I want to see in Elemental is a significantly expanded version of the trade system in Colonization.
For internal trade, Colonization used an excellent supply/demand system. My only problem with it was that the control interface for it was clunky and confusing. I think that system should be adapted for Elemental, with a few changes. For one, because it seems caravans will be automatically generated in Elemental, there needs to be priority settings to determine which settlements are supplied first.
I posted this in the Elemental Subsystems thread, but I think the topic deserves its own:
I don't think trade should be instantaneous, but it should use the caravan system. If you just trade for a specific amount of a resource, a caravan should set out from one end and make its way to the other. If you set up a constant trade route, caravans should constantly head back and forth. This would make trading with close nations easier due to both distance and danger. A caravan coming from far away will be much more likely to be raided than one coming from your next-door neighbor.
I also think that foreign caravans traveling through a nation's territory should generate some income for that nation. This would actually provide some protection for caravans - if you raid every caravan that comes through your territory people are going to choose a different trade route and you will lose that secondary income. However, if you allow all caravans to pass through your territory, even those heading to your enemies, you could gain a reputation for being gracious. The AI could calculate the safety/value of different trade routes and nations' reputations regarding foreign trade could be a major factor. So if you're in a central location, or between two major powers, it might be worthwhile to allow trade to pass freely through your lands and become a major trading hub. If you're hostile to trade, routes might be diverted around your nation, sacrificing the shorter route through your nation for a safer, longer one.
Basically, simulation of major trade routes and associated benefits would be amazing. For this to work well, trading would need to be a major part of the game - which I think would be a very good thing. Resources should be dependent on terrain, and abundant but scattered enough so that most nations will have a surplus of some things but shortfalls of others, making trade very mutually beneficial.