Yeah I have a number of ideas floating around here like this. I'm kind of a brainstormer

. Upon more thought I think that a 500 BC to 1500 AD would be better. Focused on Europe, Africa, and the Near East but subject to Asiatic depredations. China and Persia were within striking distance of each other. Even if the western parts of China (Seres) is off the map there's still the possibility they don't get halted by the Abassids and steamroll the Near East... Nothing says alternate history like the the Chinese invading randomly. At the very least the Chinese factions would be crucial for introducing world trade. Certain border regions would be absolutely filthy rich from opening up trade routes. Or perhaps events could unlock Exploration and Trade Guilds. I'm sure if Rome lasted until the renaissance it would have begun colonizing the West and East Indies.
The factions would be highly generic and open to surviving and adapting throughout. For example The Greek Factions are Graecia, Macedonia, Thracia, Seleucia, Aegyptus, and Roman. Greece can be the Delian League Hoplites in the Classical Period, Magna Graecia in the Hellenic Period, Greek Romans in the Roman Period, Byzantines in the Dark Ages, one of the fractured Byzantine successor states in the Middle Ages, or whatever. So they can reemerge to suit their times or evolve to suit a new reality. Would the Romans lose the Punic war, survive as a smaller state, and eventually evolve into being a Gothic armored naval power of "Venice" or "Genoa" or would they dominate the entire map? That's up to the player.
Greco-Roman Culture (Italic and Hellenic Peoples) 6
Graecia Greek Faction
Macedonia Macedonian Faction
Thracia Thracian Faction
Seleucia Hellenistic Faction
AEgyptus Egyptian Faction
Roma Roman Faction
Semito-Libyan Culture (Saharan and Arabian Peoples) 3
Carthaginia Punic Faction
Mauritania Moorish Faction
Arabia Arabic Faction
Afro-Ethiopian Culture (Subsaharan Peoples) 1
Ethiopia African Faction
Indo-Scythian Culture (Persian and Scythian) 4
Persia Persian Faction
Parthia Hellenistic Faction
India Indian Faction
Sarmatia Persian Faction
Sino-Turkic Culture (Oriental and Steppe) 5
Hunnica Turkic Siberian Faction
Serica Turko-Chinese Faction
Sina Chinese Faction
Tartaria Turkish Faction
Mongolia Mongol Faction
Germano-Slavic (Germans and Slavs) 4
Gothica Gothic Faction
Dacia Romania Faction
Lechia Polish Faction
Ungaria Hungarian Faction
Germano-Gaelic (Germans and Gauls) 7
Britannia British Faction
Anglia English Faction
Saxonia Saxon Faction
Francia Frankish Faction
Burgundia Burgundian Faction
Franconia German Faction
Iberia Spanish Faction
But obviously this would require quite a bit of work even if it was able to borrow a lot from other mods covering the given periods.