Improving the tradesystem with currency
Instand of "exchange" good for each another which limit the player to only 1-2 deals it would be better if different city offer different things that can be buy/sold with a currency. To make the entire system more interresting the player could investe materials in a city to expand there materials there offer. On that way the player can get materials that he cant currently product or have a shortage on. (like coal for example or iron).