And I definitely understand where you're coming from, as the "medieval Europe" thing is more a historical and stylistic feature than a mechanical one.
When you say ancient world, what kind of thing do you mean? Are we talking Xena, or the Iliad, or something else?
The smear of Empires and Kingdoms that dotted the world and contended prior to the rise of the Roman Empire, where Babylon, Egypt, Greece, the Trojans, Assyrians, Thracians, Warring States Chinese, and whoever else I want to throw in there, even if they were separated by hundreds or thousands of years, living and striving and trying to kill each other, all the while occasionally getting eaten by the horrible things that live in the blanks spots of the map.
So a little like Xena, but more like Howard Conan, and Lieber's Nehwon. Nehwon is often thought to be a Renaissance world, because his sword fights often resemble fencing duels (Lieber was a fencer), but if you look at the dress, weapons, technology, and more tellingly, the alternate Earth that was a clone of their own, Nehwon was at most, early Roman.
EDIT: but even that doesn't really require separate rules systems (even the armor can translate to a myriad of periods), just different descriptions, and possibly different art.
