Hate to bring it up again, but this:
it's in between the Tigris and Euphrates, surrounded by Havilah (Hebrew for "expanse of desert") and south of Kush.
Just wanted to say your geography is a little distorted. Kush is on the Nile River south of Egypt. http://upload.wikimedia.org/wikipedia/commons/6/6c/Kush-400aC.jpg
On Imperials:
I think a range of skin colors is good, perhaps with some differentiation between Nibenayans and Colovians. They should be of one race though. Skin color should center around a olive or brownish complexion, with dark hair and dark eyes.
The heart of Cyrodiil should be temperate rain forests, like the Northwest Pacific Coast of the U.S. and Canada. Heading west towards Skingrad, Cyrodiil should rise into a plateau, with cold, frosty, and dry winters and blistering hot but wetter summers. Think something similar to Tibet. Here you'll find herders and small salty lakes dotting the landscape, with civilization keeping close to water sources to irrigate their crops. Just before you reach the coast, the plateau drops sharply into a Mediterranean-climate-like region in which you find Anvil and perhaps Sutch.
Going south from the Imperial City, you should encounter marshlands, bogs, and extensive slow moving river systems (as opposed to the waterfalls and rapids of western or northern Cyrodiil.). In parts of this region, there should be extensive sub-tropical or tropical rain forests, even reaching right up into the lower Valus Mountains, where perhaps Mir Corrup stands, jutting above the tree tops on a cliff face and overlooking the lower Nibenay. Just to the north, we have Cheydinhal, with its extensive pine forests, and to the North West of here, we have marvelous deciduous forests all the way to Chorrel.
I would sort of design the Imperial style with Latin and Iberian influences, mixed it distinctively with Meso-American, Turkic, German, and other influences. As I see it, an Imperial soldier would be an awesome combination of http://www.civfanatics.net/uploads8/002_Samnites.jpg, http://www.sbceo.k12.ca.us/~vms/carlton/Renaissance/Aztecs/AztecWarriors3.jpg, and http://www.kismeta.com/diGrasse/Costume/Hungarian/19dw2.jpg.
Man, just writing this post totally makes me want to make a mod for Oblivion: Cyrodiil. :hubbahubba:
EDIT: I'd also like to see some Pacific North-West Amerindian inspiration as well.