As some posters may have noticed I am really into trade and economics in 4X games. The lack of trade or luxury resources in Elemental has always bugged me, but Elemental does not have happiness and I wouldn't want to see a direct ripoff of the Civ system anyway. However lately I have been thinking and reading a lot about trade and suddenly an idea popped into my head. 

Luxury resources could very simply provide a bonus to the caravan gold multiplier on all traderoutes to the city that has the luxury resource in it. This would not only benefit the city with the luxury resource but all the cities with trade routes to it. This would instantly make any city with the control of a trade good the hub of a commercial network because you would want to send all your caravans there. Even other players would want to send their caravans there in order to get the bonus. Having other players connect to the city would not only help them but would benefit the player who controls the city because the city would gain huge gold multipliers from all the caravans.

A smart player would also specialize this city towards gold production. Resulting in new type of city: the trade city. Players would be sending caravans from across the world to this city.

You could also add even more depth by instituting trade pacts that allow players to control which players can send caravans through their territory. What, you want to send a caravan through my territory to that trade city controlled by Alter? Too bad, first you have to pay me for a caravan passage treaty.

No joke this system would be far superior to the Civilization system because trade goods aren't teleporting around. And this system would be so simple to implement.... just add resources that boost the caravan gold bonus. If nothing else I would like to be able to mod this type of multiplier in myself. The only change to the AI would be to have them prioritize sending caravans to cities with the bonus.