PIRATE EXPANSION!
In a Pirate Expansion they can make looting a skill, make ships armed with cannons, place armor on your ships, purchase additional cannons, take over islands and make then un-approachable by sea. Perhaps a new building called a Fort. In addition have fishing Ships that get special bonuses for fishing and treasure hunting, but only armed with one or two cannons. Would love to have the warfare version of the game Pirates of the caribbean placed into Ultima Online!
This list can go on and on...!