Rather than implement NPC trader ships, I think it could be much more interesting and inline with SoT to implement systems where PLAYERS can be de facto traders with other players and are sometimes incentived to be. e.g. players could find/purchase more per session utility items (e.g. like the crates/cages we have now), and thus trading those to players for treasure/other utility items could be something that could reasonably happen. This could be accomplished simply by adding more utility items, of which there could be a whole slew possible
An idea I just had for one might such utility item might be something like a safe or lockbox. Essentially a container that, once placed on a ship and 'clamped' cannot be picked back up and can contain a single piece of treasure loot. Only fellow crew members would be able to 'unclamp' and pick up the safe. The loot would be 'locked' in the safe, but could be 'cracked' by enemy players through playing some form of simple minigame. It could also be opened through being hit by a gunpowder barrel. Fellow crew members could retrieve the loot simply through interacting with the safe for some amount of time (similar to how it takes time to repair a hole). If a ship sinks, all safes would despawn along with the ship, releasing the loot item to float up to the surface just like any other item. Safes would not be persistent across sessions, like all utility items ought to be. This possibly isn't the most useful idea, but it shows how other utility items could be implemented in the game