Hi there wolflance! I support your civ. proposal at 100%! I would love to see khmer armies in an RTS. For the late game, I would think more about a mounted elephant unit (with archers and flamed arrows), and maybe an elite infantry guard unit (apsara guard?) reflecting the fanatism and devotion to their king. In the last centuries, the Khmer Empire had been severely weakened by the loss of vast territories (notably in the western part, with the uprising and progressive independance of siamese provinces) and the loss of its capital in Angkor Vat at the hands of Sukhotai...I think having champions like royal guards or monks should reflect the defensive_yet combative_ stance assumed by the political power during these troubled times...
I can already imagine the magnificence of decorated war boats with garuda shaped or naga shaped bows, majestic temples with gigantic towers, fierce yet elegant warriors, etc.
Here are some eye candies:
http://www.devata.org/artistic-visions-of-angkor-by-maurice-fievet/#.V9Fv1mBkiM9