Give primary/war-ship a purpose after map fully explored

Once fully explored there is rarely a need to crew up this strong ship. Create a world map above the current world map to allow you to send your ship on broader missions. Fight pirates, join coalitions, random-action-x. In return the ship could bring back small amounts of expensive/useful things.

Or participate in humanitarian missions where you need x-amount of medical supplies and people (As doctors) for a period of time.