I have come to the conclusion that this game needs the Sumerian civilization. I am always hearing about them on the History channel and other programs, and want them in for this reason, and because they are so interesting.
That aside, wasn't Sumer also the first Civilization to discover writing? How could they not be in?
Unique building suggestion: Ziggurat.
Unique Ability: First to Writing (provides great scientist on discovery of writing)
I believe they are already in the Wonders of the Ancient World DLC. Why not make them a playable Civilization in the rest of the game as well?