Both Elite and Patrician had fairly simple algorithm economies that responded to supply/demand.
If town A produces commodity X, then the price of X will rise if lots of traders buy it, and fall if they don't. If town B needs X the price will rise there for that commodity if no one is bringing it in, but fall if everyone brings it in and there's a glut. The clever bit will be using AI captains to take up the slack until the player population grows and establishes its own trade routes, and then wean the economy off the AI.
If you're prepared to allow players to fail, then trading becomes much more interesting. Allow to develop player-controlled trading companies loosely based on the East India Company, other chartered companies or the Hanseatic League to replace the guild/clan concept. Although these do require the concept of player owned property - ships, commodities, warehouses, company offices and guild chambers. If the economy is set right, the missions will create themselves.. the route between A and B becomes known as a profitable trade run, and an equally profitable run to ambush by brigands.
For ship ownership, new players are apprenticed to a trading company or faction (the apprenticeship being the duration of the initial training missions). This leads to a loan to pay for a ship (to be paid back to earn Free Captain status), or the narrative could contrive for the player to be left a small ship by a distant relative or be granted a ship as a reward for services rendered.
I could foresee a small trading company with three cargo ships and three escorts, plying the trade routes in pairs.. whilst two scout ships explore the map to find new routes and trade opportunities.
Be sure and include something like the Droknar's Forge run from GW, a convenient shortcut available about half-way through the game but only passable at extreme risk by a well organised fleet.
