If we treat every Indo-European civ as Proto-Indo-Europeans in the Neolithic, the Pontic Steppe is going to be even more crowded on TSL maps than the Balkans are in Civ6...as every European civ + Persia + India (+ Hittites, if they're included) starts there.
I think it's obvious that no group prior to 4000 BCE is playable in Civilization: we have only traces of languages, and most of those reconstructs from later iterations, no native site names, let alone cities, and the only thing for 'Leaders' would be the occasional, usually much later, Mythologcal Founders.
So, my concept (at the moment, I ma post something more evolved tomorrow) is that you start as a random 'tribe' and this being Civ and not That Other Game, you personalize it by making up a name or selecting from a list. Quite possibly do the selection of a name after you see your starting Position, so if you are near a large river in a grassy plain you might pick:
Yamnaya
Botai
Repin
Usatova
Khvalyna
Shulaveri
Or you could set up the game as "I want to (eventually) play China" and choose from:
Daxi
Majiabang
Xinle
Houle
Cisha
Peilagang
All of these are simply names of early cultures given from archeological sites, which is why the option to name your own should be included: in no case do we know what these people were actually called or called themselves.
Now, you could immediately settle down if you've got a good site: domesticatable plants, like Korn (any of the grass grains like Wheat, Millet, Barley, etc), Rice, or Potato, a river or coast teeming with fish, a marshland teeming with waterfowl, a plains covered with huntable deer/gazelle/antelope or domesticable Sheep, Goats, Cattle (Goats and Sheep would actually be on Hills) - in other words, enough food to support a larger population than Wandering Extended Family. In that case, you get to choose a Civ to play right away (because, yeah, we'll stick with City = Civilization for most playable Civs) - but, since every decision you make in the game should be a Trade Off, you lose the opportunity to Scout Around first - because until you've got a Hunter working one of the tiles with animals, you cannot form a Scout to continue exploring.
You could also settle down without choosing a Civ, but in that case your 'settlement' will not grow into a City, it will simply spawn a new Wandering Group whenever its population grows to 2. This gives you the chance to keep exploring and settling new territory, but no chance to build up the Culture/Social points that the specialization in an urban setting would get you - again, always a Trade Off of some kind. But even the non-Civ group can (slowly in the early period) get Science progression, and so reach Domestication of Cattle and/or Horses so that they can choose a Pastoral Civ to 'play' towards the end of the Early Period (4500 - 3000 BCE). Because, of course, we have historical pastorals that can be made into Civs for the game: Scythians, Xiong-nu, Mongols, Lakotah, Comanche,, Huns, etc.
Basically, I want that Early Period to be much more full of Meaningful Decisions about when and where to settle down, and how you want to progress and even what to Research compared to the rally Bare Bones Neolithic that HK gave us.