http://www.imperial-library.info/sites/default/files/pge01_cyrodiil_art_02.jpg, http://gravitybong.net/files/public/1264360115_371_FT0_morrowind-screenshot.jpg Roman. This might be surprising, but there were plenty of people that conquered territories with professional armies and set up provinces. Imperials use some Roman-sounding names and borrow some of their armor, but just about everything else is not (specifically) Roman. Even the word "imperial" is an innacurate simplification - do you mean the Colovians, the Nibenese, the people of Anvil? The Nords are a mashup of pre-christian Vikings and classical Hindus, with a dash of wasabi thrown in for extra flavor. I'd even say they've got some Amerindian and Jewish references.
http://www.uesp.net/wiki/Morrowind:Imperial_%28style%29 http://www.jwaller.co.uk/nas/images/higham_ferrers.jpg http://www.uesp.net/wiki/File:MW_ImperialWeapons.jpg. With castles and gothic cathedrals borrowed from the middle ages. There were no differences between the Colovians and the Nibenese in-game, which is what I'm primarily discussing.
If it looks like a Roman, sounds like a Roman, and acts like a Roman, its probably a Roman.