I would love CK2 to cover all of Eurasia and northern Africa, but only if paradox could find game mechanics for preserving a realistic feel of who can go where. Mongols attacking Southern China and then hitting Venice 20 years later? No problem--that could easily have happened IRL. Veldmaarschalk's example of Edward, Duke of Saigon? A bit lame. I'd want to see logistics mechanics that would ensure that we wouldn't see crusaders invading Tibet.
From a game-design perspective, there would also be the tremendous problem of finding game mechanics that could transmit the historical feel of not just Northern Europe, but of the Empire of Mali, of Persia, of the Khmer Empire, of the Eurasian Steppe peoples, etc. You'd almost need a dozen or so different sets of game mechanics to cover the different regions of the world.