You'll recall that that empire had some characteristics that made it imperial, like a unified system of government, continual military expansion, and a diversity of subject peoples. The Roman Empire had all three of those characteristics long before it became the Roman Empire. Like Rome started out as a city and then it became a city-state and then a kingdom and then a republic, but that entire time it was basically comprised of the area around Rome. By the 4th century BCE, Rome started to incorporate its neighbors, like the Latins and the Etruscans, and pretty soon they had all of Italy under their control. But that's not really diversity of subject peoples. I mean, nothing personal, Italians, but you know, you have a lot of things in common, like the constant gesticulations. If you want to talk about real expansion and diversity, you've got to talk about the Punic Wars.