The Known World of Mystara is a Hyborian Age-esque fantasyland of often thinly disguised real world cultures from a variety of historical eras, but the general vibe seems Medieval to early modern. I think it would be interesting reimagine Mystara as a more ancient world inspired, Sword & Sorcery setting, though will not greater adherence to a single era. Here's how it could breakdown:
Emirate of Ylaruam: This desert region has always been oddly placed, but depending on what latitude you think it's at, it might be weird for it to be a hot desert. Maybe it's a cold desert like the Tarim Basin or the Taklamakan. You could ditch the faux Arab culture for something more Central Asian, and give it's central religion a more Eastern flavor.
Empire of Thyatis: Less Byzantium and more Rome, though I would probably move it more in a Hellenistic direction. What the Empire of Alexander might have been like if it had been able to hang together better after his death.
Grand Duchy of Karameikos: This would stll be a breakaway, former province (though not a "Grand Duchy"). There wouldn't be true, Medieval feudalism here, but something more like the Roman latifundia.
Kingdom of Ierendi: This kingdom ruled by adventurers is kind of a pure fantasy trope, but I would give its material culture a Minoan spin.
Minrothad Guilds: A plutocratic thalassocracy more like Phoenicia or Carthage. The Guilds would be collegia.
Principality of Glantri: Well, still a magocracy, but maybe more like the Estruscans?
Republic of Darokin: Keep the plutocratic republic, but cast it less as Venice and more as Republican Rome with a of the "center of caravan routes" feel like Samarkand or Palmyra. A bit of Persian influence wouldn't be misplaced as Darokin does border Sind, which is sort of Mystara's India.
The Northern Reaches would probably still just be sort of Vikings, I guess, maybe more proto-Vikings like the horned helmet wearing raiders of the Nordic Bronze Age. Ethengar might be more Scythians than Mongols. Haven't given much thought to the demihuman lands or Atraughin.
