I think it would be pretty cool to have a seperate game client dedicated to playing offline.
Your pirate DOES NOT cross over so its a completely seperate experience but basically you can play SOT in areas where there is no internet and still enjoy the game. Perhaps you are on a plane, camping or just staying in a hotel with horrible Wi-fi.
Its not just a carbon copy of the game though! It has roaming npc ships both passive and hostile and even faction ships you can join in with.
You can also hire npc crews who man cannons, repair and bail and all the tall tales and voyages we have in the main game.
Id expect to pay for this and I for one would be happy to. Id still play online more than offline but id certainly use it regularly. I work away and im 50/50 home so something like this would be ideal.
You could add Co-op so you can play with friends too. Good thing is it wont affect the main online game as the pirates are not interchangable.