i would like to have civilization acknowledge that no, not everyone was a full-fledged civilization in 4000 BCE, and that some peoples didn't even have a cultural identity yet. i want some civilizations to start and stay as rural illiterate barbarians for dozens or hundreds of turns while others start as fully urban civ, and make that fun and balanced. to help this, it could help to decrease the focus on cities and focus more on the small scale: towns, villages, and hamlets. ideally, there would be a 'developing civilization' game mechanic, where to allow for the diversity of civs and leaders civ allows, you would start with a base culture and then evolve into one culture depending on your environment, history, and actions.
For instance, Indo-European (start in steppes): migrate to a rocky island lying nearby, become Myceanean Greek, get a leaderhead and some abilities. then, as the bronze age collapse happens, turn into Classical Greeks, get a new leaderhead and new abilities. then either survive being conquered or unite as a New Greece or as (Eastern) Romans.
t there could be shared cultures, there could be culture splits. These cultures would be invariant of Era unlike in Humankind. There would also be a cultural influence mechanic (like if the Celts or the Illyrians united Europe instead of the Romans.) that would boil down to a few bonuses inherited from the other leader. So, each Civilization would have 1-2 empty ability spots for other civilizations to fill in.
here are some examples to make my point.
Indo-European: Splits into Italic, Greek, Baltic, Balkan, Slavic, Celtic, Iranian, Hindi, etc...depending on place of migration. As an example, a player would choose Polish when they get voluntarily converted to a new religion, coming from the Slavic predecessor civilization, whereas Roman would come from waging many wars in a hilly peninsula.
East Asian: Splits into Japanese, Chinese, and Korean. For instance, one would become Japanese if they migrate to a rocky island. To select Chinese, move to a region with fertile rivers surrounded by hostile land (mountains, desert, ocean)
Central Asian: Splits into Mongolian, Tibetian, and Turkic. If you want to play as the Mongols, migrate to the steppe and become familiarized with the horse. If you then conquer China, you become the Yuan Dynasty, which the Chinese can also become if they get conquered by you.
Middle Eastern: Splits into Egyptian, Mesopotamian, Anatolian, Caucasian, and Arabian. If you want to play as the Egyptians, settle in a fertile river surrounded by desert. If you want to play as the Mesopotamians, settle between two fertile rivers surrounded by desert. If you want to play the Caucasians, settle near mountains and a lake or sea.
You could also become 'civilized' by taking over another literate urban civilization as rural illiterate barbarians, like how the Germanic tribes did. In the current Civ paradigm you cannot replicate the Fall of Rome, nor the Indo-Iranian migration into India, nor the urbanization and education of Scandinavia. There would be, say, 64(!) civilizations to play with at the minimum.