I would suggest natural boundaries. The Urals work pretty well for Russia, and nothing was known of south of the Canaries, but I agree that Iceland needs to be fully portrayed and why not Greenland and (pushing it) Newfoundland, because that was part of the known world, too. I would like to see a little bit more of the Red Sea and Persian Gulf, though I don't know how far down we should be able to go. It might be fun to throw in Ethiopia, with an event "I found Prester John!" Regarding Persia, I would push things back a little further east, but also simulate Turkish and Persian migrations into the game world somehow. The Seljuk Turks had more territory than is portrayed in game now, so as they are a major enemy for Byzantium and the Crusader States that should be taken into consideration.