I'd honestly love a pre-historic era similar to Humankind either as a game mode or implement it for Civ 7 where you would essentially have to discover agriculture, in order to build your first city.Well the other side of this is that Civ isn't really a good simulator for pre-historical stuff. Like it has the pretense of doing so, with development of agriculture and so on (in some versions, in others you start with it). But a city of 20,000 people, which is usually the approximated population of a pop 1 city, has already developed a lot of those things. Maybe if civ had basically an entire pre-historical era, then you could fit in some of these groups and ponder what could have happened.
I don't know if that's the best way to implement civs such as the Harappans, Olmecs, or Norte Chico being playable though. I think it would make playing nomadic steppe civs like Scythia or the Huns more realistic instead.
Having a known language does matter as long as you continue to implement voice actors for the various leaders.I do however agree with the above poster that having the leaders is essential - and that civs with no known historical leaders just can't fit. Language...well, I'm not so fond of it, but it seems to be here to stay also. So yes, people for whom we have no documented language or no documented leaders are....pretty much just not possible. Fortunately, we do have surprisingly extensive documentation and a surprising number of fairly recognizable figures.