The intentions behind this proposal:
1) Cause more action ("combat") in the game
2) Make it more important to be be Duke in a port-holding nation
3) Introduce more complexity to trading
So, the main idea is simply to change the way supply and demand works - particularly demand. Today, the ports that has least of a good in their "secret warehouse" is offering the highest price for that good. And the one having most of it's "own" good is selling for the lowest price. So this is a sort of "relative" supply and demand. I suggest to replace this by an ABSOLUTE supply and demand. This means the following:
A) A port producing a good, is selling that good for a price solely dependent on the amount of that good in his warehouse, independently of other ports. The price has a minumum and maximum value, with the minimum price being much lower than today (the maximum stays the same). (In addition, the price could also be dependent on the "port warehouse size", which could be dependent on port population - this is optional).
B) A port is offering a price for each specific good based on how much it currently has of that good in it's warehouse - independent of other ports. The price has a minimum and maximum value, with the maximum being MUCH higher than today (maybe 15 gc above minimum), while the minimum stays the same. Goods are consumed at a fixed rate (that rate may be dependent on port population; again, optional).
C) Party cards change. Rather than "sell" goods at a fixed price, they now "sell" them with a certain percentage bonus. Maybe 50% over what you would otherwise get in the port. This means that Duke bonuses and East india card bonuses also are boosted by party cards.
D) When successfully plundering a ship, the cargo of the losing fleet is lost. It's value is transformed to gc and given to the winner as gc.
Points A and B will give more complex trade. And the increased max prices will make even long distance trade routes profitable. Point C ensures that being Duke is worth fighting for. These three points also gives more income to traders - if left undisturbed. Point D will make plundering slightly more profitable, and also make it slightly easier to steal trade ships, since they can no longer use cargo to pay ransom. Also, it makes it possible to prevent a fleet from getting the cargo to it's destination.
So, how and why will this cause more action? Well, up until now, traders have had no reason whatsoever to fight each other. A trader gets more or less the same from his trade routes, independently of how many others use the same trade route. However, with these rules in place, that changes. It now becomes profitable to stop other traders from selling a particular good in the same port as you do. So there's an incentive for traders to get other traders plundered. It will make for an interesting mix of trade and combat. Perhaps, for instance, Egypt is fighting (or paying others to fight) any non-Egyptian trade vessel transporting tobacco to Goroum, to make sure the prices there stay high.
I think this will give an incentive for traders to wish to stop other traders, while today, there is no reason for that. This incentive will, I believe, cause more action in Avonmora.