Now that we know all cultures from the first three Eras, we can also say with certainty which ones will not be in the base game and are good candidates for expansions and DLCs.

Jeff Spock has mentioned in a past interview (which I can't remember the link and would be glad if someone could post here) that Amplitude is still not sure if the route of future content will follow the path of, let's say, a whole Medieval pack or a whole Asian pack or something distributed through eras and regions (I'd say this last would be the most interesting and likely choice).

Considering that, which missing cultures from the first three Eras do you think are most likely to (or should) be added after the base game?

My guesses are:

Ancient Era (please note that I know very little of this era and its cultures):

-Sumerian (Agrarian?);

-Minoan (Builder, Aesthete?);

-I would like to say Etruscan but it seems they are way ahead.

Classical Era:

-Scythians (Militarist)

-Han China (Expansionist)

Medieval Era:

-Kievan Rus

-Song China (Scientific)

-Japan

-Holy Roman Empire

-Cuzco Kingdom

-Anasazi/Pueblans (Aesthete)

-Tupi-Guarani (Agrarian)

-Mali (Merchant, although except Egypt all african cultures by now are Merchant)

-Benin

-Songhai

Honestly, I hope that future additions to the game will be driven by gameplay (or gameplay potential) rather than by whether a culture is deemed important enough to be represented.

Looking at what we have currently, it looks like there's really little that actually differentiate cultures with each other (the culture type seems to matter much more) and I fear that the game would be very repetitive with no real gameplay difference between a civ and another.

I think it's clear that in 4X players prefer civilizations with clearly different playstyles (such as Venice in Civ5). Humankind looks like a min/maxer paradise, so I hope new content will be focused on variety rather than adding more of the same.

I would like to see a new affinity, religion, which is an important part of human history. I'd love to have Tibet as a religious civ, for example.

I think the Polynesians in either the Ancient Era, or the Classical Era, would make for a very unique addition to the roster. Perhaps they could be Aesthete's with bonuses focusing on navy. One thing I know for sure is that if the Polynesians ever get in, the artwork is gonna be amazing! :D 

For the Ancient Era I hope Sumerians, Akkadians, Elamites, Minoans and Tyrsenians.