I've been messing around with the two Mesopotamian civs. Are they really meant to be that pathetic? There is no way of getting reasonable production...
In the upcoming "expansion pack" version, I think you should consider changing one of the Mesopotamian civs to something else. They are both weak Agricultural civs with early UUs in the same overcrowded area.
There are a number of worthy civs that could take the place of Sumer or Babylon. In East Asia, the Siamese or Khmer could be added. In the Middle East, Israel could be good, especially if religion has more importance in the mod. R8FXT made an excellent David animated and era-specific leaderhead. In Africa, you could add any of the great West-African civs. (the Mali come to mind) You might not want to do that because it would discourage the colonization of Western Africa, which is key for the European civs.
Any of those civs would be good. But without a doubt, the civ I think most needs to be included in the mod is Poland. Firstly and most importantly, it fills the map out very nicely. Germany and Russia are both given a lot of expansion room in Eastern Europe, even though they are both already extremely powerful. It also gives Russia a closer neighbor to think about. Secondly, there are excellent leaderhead and UU graphics for Poland. Thirdly, it would be great to have a true Slavic civ in the game. (Russia is more of a mix of Slavic and Varangian culture.)