The are many parts to Pirates glory and they are connect through player interaction however one of the basics of the game trading isn't.
I would like to suggest the creation of trading houses ie Merchant houses that inhabit ports. One merchant house to each port.
These merchant houses could interact with ruling country by wanting lower taxes which could be pushed by lowering that's country's player influence in their port, or more plantation licenses being given out.
They could interact with plantation owners. Not a plantation owner so I am uncertain about there specifics. But the Merchant house could provide some of the requirement goods for a percentage of the product. or outright buy some of the product.
It could also tie in with pirates where merchants house could have itself a small bounty board where it offers a gold reward to attack
1)competing merchant houses in other ports, or
2)players who are competing merchants, ie using their ports location and buying the goods there.
3)Also to attack pirates attacking its merchant fleets ie make certain NPC merchant fleets belong to Merchant Houses
Where each port has a merchant house that can give you benefits for joining them.
1) They could you give you a slightly better price if you ship ship over a certain amount of goods per day from there port
2) Limited mission where a certain port would pay large amounts for the goods their port produces
3) Limited offer to buy goods at high price Ie the Merchant house is building a lot of ships wants more wood and iron
For these benefits there should be cost. One of those cost should be requirements to join them
Ideas for Requirements to join
1)Pay a fee every time the prices change,
2) Have an influence cost or requirement,ie the Merchants wants to use your connections
3)Buy goods consistently from there port, ie The merchant house needs to have 10000 goods bought daily from there port
4)Not be in another merchant house
Another potential cost could be rival merchant house of other ports
1) they could sent pirates after your fleets via their bounty boards
2) Raise the cost of goods bought in there ports or lower purchasing price.
I personally want a feature similar to this to make the markets and being a merchant more interesting. Since it would give merchants another layer to trade route optimisation.