...was actually a mixture of Asian mythologies. *shrug* Kinda seemed like a no-brainer.
Either that or a culture based on Native American lore.
I'm not sure why these wouldn't be implementable cultures, unless there is simply too much fear of a backlash from the groups that are concerned about "politcally correctness", especially here in America from Native American tribes that still adhere to the ways of their ancestors.
Both paths would allow for a rich diversity of units.