Resources grown at the plantations should be available as trade-able resources in the game. Resources automatically sold to the port feed the availability of the resource for sale, and sale value determined by the product's supply in the game and the port of sales distance from production points.
Nations and governors would have to determine what crops are most valuable for their port's location when determining permissions.
A port producing the same product nearby would cut into another port's profit and could lead to action to reduce the opposing plantation's production of a given resource, thus plantation wars.
I would naturally assume ports and nations profit from taxes on sales and purchases of the plantation resources, so keeping the resource at max value would be a priority.
Another post addressing the desire for new trade resources could be addressed through plantations in the manner I have shared here.
Contract shipping could also be an interesting dynamic with regard to certain plantation products and worked into nation missions involving plantations.