I've never played Humankind, so no real clue what the gameplay there is like in general or in the Neolithic era. Thinking of what Civ having such an era would/could be, like, however, and thinking almost purely from a game design perspective (since I'm not an anthropologist)...
1.) It would need to be short. Like, probably 5-15 turns. Even knowing little to nothing about how other games may have handled this, I can agree with others here that any extended period where we're not playing "the real game" or "as your chosen civ" would get old fast.
2.) It would need to serve a legitimate purpose, and as far as I can tell, the best reason to have it would be so that you can get a better idea of what your surroundings are before choosing where to settle your capital. We can technically do this now, but the opportunity cost of doing so is massive, so I think this would have to look like a small amount of opening turns where you are "earning" your first settler, essentially. Maybe via finding//pillaging resources? That's the best way I can think of right now.
3.) It would be very tempting to give one or more civs some sort of bonus in this era, which I think would be a huge mistake. Early era bonuses can already snowball quite a bit and any bonus that translates to "found your capital several turns before anyone else can" could easily turn out to be broken. But on the other hand, you'd need a real goal in this era beyond just waiting things out before the turn when everyone can found their first city for the era not to just feel annoying and perfunctory. Which could get tricky in terms of start biases, some of which might naturally tend to start a civ nearer to immediately pillageable resources than others. But in any case, a bonus such as Portugal's or Gran Colombia's shouldn't kick in until their first city is founded, I think. (As in, nobody's should under this system.)
4.) As the game has become more tied to understanding the map and how it will affect your city-placement (which obviously jumped forward quite a bit in Civ6) the AI's ability to choose a good place to found their cities has not kept up. As much as I'd honestly really like a few "mandatory" turns of exploration before selecting a founding location, it could set the player that much further ahead of the AI at the start.
5.) Any mechanic by which I can imagine the first city being settled feels imperfect to me, at least so far in mulling this over. The two big options I can imagine are either earning a settler on a tile on the map where presumably one of your units already is (or next to it, or whatever), or else just choosing a tile you've explored (which is legal for city placement, obviously) once the time comes. The latter is off because it's immersion breaking - it works in CivBE because you're landing on the planet from above and so it makes a degree of sense, but what would it represent here and how would the game represent that in a satisfying manner? But the former is worse to me, because if you spend 5-10 turns exploring, get that settler on whatever tile you're on at that moment, you've just recreated the "problem" that having a Neolithic era is supposed to solve, since once again there's a massive opportunity cost for not just settling on that spot.
5a.) So maybe, to split the difference, you start with two scouts/warriors/whatever. Two identical units, in any case, with basically three abilities. They can attack/defend/fortify until healed, they can explore/exploit/pillage, and they can "settle in" on a tile. If one of them chooses to "settle in" on a tile, the other cannot while the first is doing so. The "settle in" option becomes like a sort of project (picture workers in earlier Civ entries taking a number of turns to improve a tile) but the project can be sped along by the actions of the other one, and starts off with a number of "points" already earned based on what you've done prior to starting the project. Basically, two units, both of which are exploring, one of which will become your first city, the other of which will upgrade to your first warrior once that city is founded. When you find a good location with one of them, it takes a few turns to settle in that location, with the exploring and pillaging or what have you that you've done prior to that point counting towards that project, and the exploring/pillaging/what have you that your other unit is still doing during that project also counting towards it. For opportunity cost balance, I think the exploring/pillaging/what have you would have to provide greater windfalls for the project than the PPT for running the project itself, but you'd still want to get it started quickly.
6. For all I know, this is basically what HK did, and it sounds like their version didn't work too well. But my main point is that if Civ were to do this, it would need to solve an issue (and I think there's at least one issue this could address, though not necessarily a major one) and the gameplay during that time would need to be exciting and engaging rather than kicking that issue down the road a few turns while you're waiting for "the real game" to start.
