I'm bad at be being topographically correct.
Although I know that the Arabs, Persians, Ottomans, Byzantines, Babylonians and Sumerians were completely different ethical groups, civilizations and cultures, I just feel their empires, at their height, consisted of mostly the same areas. Mesopotamia, Turkey, the middle east, etc.
Although all of these civs were very important in their time and we certainly should include a couple of them I feel that with the Persians, Ottomans, Arabs and the Babylonians in the game, we don't also need the Sumerians, Byzantines and maybe even the Hittites.
Therefore, I
rather see some important civs from other regions, like South-East Asia, Africa, South-America, etc. The Khmer, 'Nam, the 'Polynesians', Ethiopia, maybe even the Berbers or the Moors.