I wrote another article on gravely underused game-idea, the 4th in the series.
Everything outside of that gives you a chance to make a game unique. Anno 1701 introduced slight Adventure/RPG mechanics: the player is issued quests which they much complete using their player avatar (their ship), while other compete with them for rewards. In Anno 1404 these became unique items that could buff the player’s islands or ships.
Go check out the rest on Tuts+!