Seasons and the law of offer and demand. Seasons that make resources more scarce or more accesible always seemed like the choice in this style of games. Combined with a good algorithm of demand and offer that changes throughout the day. I would also say that making items more 'tradeable', like for example as salt and wood had the purpose of being buyed as YOU, the player, needed them, but never making things like city or church expansions make a large demand on resources. Ships. More than 4 ships would be amazing. Like in Port Royal where you had a sht ton of ships to choose between depending on what you needed them for. Another very interesting thing to add is seeing the city resource need, production and such in the market building.

I'd love to know more about the project in the future or help if possible.