Didn't someone make a whole pack of Mongol mounted units?
You would definately want those. Also, there are a bunch of Chinese units (Chin Spearman, etc.) that could be included.
Of course, you have to find a balance between flavor and size. I personally don't think every mod has to have 100 flavor units to make the game more fun. Mongol, Chinese, and Japanese units could be flavor, though.