Server would spawn a "Treasure Fleet" in a random location and a random time during the day. 24 or 48 hour realtime timer.
This NPC fleet would be composed of a large number of 1st rates escorted by frigates etc. A balanced fleet would need to be created to make it impossible except for large well organized teams of players to capture this fleet.
You ask "Why would anyone attack a fleet this strong?". It sounds like madness until you see that each 1st rate's cargo hold is completely full of "gold ingots" not gold, but ingots which makes the gold even more valuable saving your crafting hours and coal for other crafting needs.
Each 1st rate would have to be boarded and captured to acquire the gold ingots in it's hold. However these ships would not be able to be captured, but only access to the precious cargo. Of course all the escorting vessels would have to be dealt with before boarding and looting of the 1st rates would be possible.
This randomly spawned treasure fleet would traverse the gameworld spawning in the west it would set an easterly course heading for the edge of the map, north to south, etc. If sailing from east to west it would target a free port to dock at along the western coasts somewhere and would have to be intercepted before reaching the target port.
Just my thoughts to spice things up a little bit.