I'm pretty sure the Imperials are based of the old Roman Empire,
Edit: grammar
Imperials are, to me, a combination of the Romans and ancient Greeks. Colovia is the "roman-esque" part of Cyrodiil, while Nibenay was the area Alessia revolt began, it was the home of the First Empire and the Alessia Doctrine. The Nibenese Imperials represent the Greeks during the Hellenistic period. The Hellenistic period ended when the Roman Republic conquered Greece or when the Ptolemaic Kingdom finally fell. The War of Righteousness and the Dragon Break led to downfall of the First Empire. Colovia resisted Nibenese rule and broke away during the War of Righteousness and was partly responsible for the fall of the First Empire.
From there on the Second and Third Cyrodiilic Empires have been forged by Colovian Dynasties. Reman I was the ruler of the Colovian Estates before founding the Second Empire and Tiber Septim served under King Cuhlecain, another King of the Colovian Estates. Before the formation of the Third Empire. The Colovian part of Cyrodiil eventually surpassed the Nibenese in terms of power, much like how the Romans surpassed the Greeks as the worlds ancient superpower.
This is my take at least.