Ship to ship combat is absolutely brilliant. Having various NPC Ships would add a much needed variety to the game. It would allow for more opportunities for players to engage in ship to ship combat (and giving crews some good practice without too much risk) while also serving somewhat as a distraction, giving other players a chance to voyage for gold in peace.
You could have...
-Various pirate ships
-East India Trading Comapany ships
-An environmental event ship like the Kraken. All of a sudden you’re overtaken with fog and you see a massive ghost ship cut through in the distance.
All of these could drop random loot. The density of these ships could be tooled with to find the right balance. Perhaps they don’t attack ships docked at islands. Perhaps they stop chase once you’ve retreated a cerain distance away. Lots of cool ideas could be played with. Bounty voyages. Perhaps, hiest voayges where you have to steel a chest off a ship without sinking it? Etc.
This sort of thing is the kind of non-cosmetic content that would massively improve the dinamic variety of this game. It also adds to the narrative of the world making it feel more alive. This has continously come up in game chat.

