Unlocking by slots can result in players that have moved a lot to chose a Culture not related at all to the position they've finally settled.
I ponder to just allow a "joker" culture that anyone could chose for the first era, independently of their settled positions on the map, in case they've not unlocked a Culture by territory, or in case you use the 8 players version of the map.
The question is which one ? (until we can easily mod a new culture)
Harappan ? but all the AI may chose it over the "unlocked by territory" Culture then.
Hittite ?
@SeelingCat @Boris Gudenuf
Let's examine the provided Ancient Factions:
Assyrians - Assur, the city/settlement that gave its name to them, existed since at least 2600 BCE, but was peopled by pastorialists who migrated into Mesopotamia earlier. The first, or Old Assyrian Empire didn't rise as a power until after 2000 BCE, so over half-way through the usual Ancient Era (4000 - 1000 BCE)
Babylonians - Babylon is first mentioned at all around 2300 BCE as a minor religious center. It didn't become a powerful city state until the early 18th century BCE (1790 BCE and later). Biggest point is that while it received invaders/migrants from elsewhere, the basic Sumerian/Amorite/Akkadian-speaking population had been there for millenia already.
Egyptians - the first Dynasty is dated back to 3150 BCE and there were settled farmers in the Nile river valley for almost 2000 years before that. The Nile IS Egypt and always has been.
Harappans - Or "Indus Valley Civilization", agriculture started in the area by 5500 BCE, the 'mature' Harappan cities appear around 2600 BCE and start to crumble between 1900 and 1700 BCE. There's no sign that the population came from anywhere else, and their language doesn't appear to be related to anyone elses' in other locations.
Hittites - Indo-European speakers who migrated into Anatolia sometime before 2000 BCE, established Old Hittite Kingdom between 1800 and 1650 BCE - so, better than half-way through the Ancient Era before they actually settled down, and migratory before that (hypothesis is they branched off from the Yamnaya and/or Maikop Cultures by 3000 BCE or so, but that's still debated)
Myceneans - the first recognizably 'Greek' culture, but being Indo-European speakers, they migrated into Greece from further north and east, probably between 3000 and 1800 BCE. The 'Mycenean Palace/Citadel" period dates from about 1750 BCE so, like so many 'Ancient' Factions, not until the last third of the Ancient Era.
Nubians - agriculture in the area dates back to 6000 BCE, and the 'Kerma" culture associated with the in-game Nubians dates to about 2500 BCE. There is still a lot of debate on what language or languages they spoke and what other languages they were related to, but there's no evidence that the population came from anywhere else, which places this Faction firmly in one place.
Olmecs - pre-Olmec farming cultures existed in the same area since about 5100 BCE, and the estimated rise of identifiable Olmec sites dates to about 1600 BCE. There's no indication at this time that the population came from anywhere else, or brought any specific cultural elements from anywhere else.
Phoenicians - the Canaanite culture that was ancestor to the specific Phoenician city states developed in the Levant from the Natufians who had been there since at least 6500 BCE, but the first "Phoenicians" aren't identified until about 1450 BCE, so, again, from the last fifth of the Ancient Era.
To summarize, if you are looking for Factions that started up somewhere else and so could be 'detached' from a specific geographical start, the best candidates are:
Assyrians
Hittites
Mycenans