Dark ages like mondblut said. To be more accurate, the period from the fall of the Western Roman Empire and the rise of Islam to around the end of Carolus Magnus' reign. It was a pretty interesting time in the world outside of Western Europe. Also would lend itself well to a game centred around Asia.
What? Taking once a lifetime opportunity and wasting it for the sake of some gooks and towelheads? FFFFUUUUUUUUUUUUU.
As far as I am concerned, the southern and eastern edges of CK2 map can be safely dropped; the existing horde mechanics (sans the retarded no-attrition cheat) are perfectly serviceable for bringing mudslims, avars, hungarians etc into play.