The amount of micro needed to maximize Caravans seemed to be one of the most hated things from WoM, and thus the devs removed them in .86. Now they only move between capitals when you have an Economic Treaty. This change was overdone and I would like to see Caravans come back as right now they have simply removed trade from the game entirely, adding to the two dimensionality of the game. Which is a shame because although caravans were done badly they had a lot of possible depth and added a lot to the atmosphere of the world. A good caravan system would really help with replayability, diplomacy, and city differentiation. Trade was such a huge part of human civilization so it is very depressing to see it removed from the world of Elemental completely.
A possible idea for a caravan system would be instead of making the amount of gold generated by a trade route a blanket 10% you could make it depend on a new local resource, let's call it Goods. The amount of gold produced by a trade route would be dependent on the total Goods in both cities on the trade route. This resource could be produced by city level and improvements like Merchants and Bazaars. You could even add natural luxury resources like in Civ. They would be resource tiles that you could build improvements on like Iron Mines that produced Goods instead of say Metal. The more Goods your city produced the more other players would want to send trade routes to that city, which would benefit you as well. Trade route spam could be prevented by either a hard limit tied to the amount to the amount of Goods in a city and/or a upfront gold cost for maintaining trade routes, so you would only build them to cities that would be profitable.
Also roads would still be produced through tech as of now but setting a trade route would also produce a road if there isn't already a direct route. To prevent road abuse, like there exists now, Economic treaties wouldn't spawn a caravan; it would simply produce the road between capitals and allow players to set trade routes to that players cities. Trade treaties could also be removed as they are kinda unnecessary.
The key to an interesting caravan system is to reduce micro and spam and make it non-essential but a interesting choice, a nice bonus, and a potential strategy. While building the Caravan unit could work I think having a button on cities that lets you set routes by clicking on a destination would be less micro intensive. Also rewarding players with resources for destroying enemy caravans and punishing players for loosing them would add a great raiding mechanic to the game. Then we could even have pirates when we get naval content!