My educated guesses are
Achaemenid (Because they might have several Persian cultures)
Aksum (could be named Ethiopia?)
Carthaginians
Gaul (if not Celts and Iceni is less likely)
Greeks
Han (Chinese)
Maya
Mauryan (India)
Numidia (could be named Berbers) This might be the one mistaken for Libyans earlier.
Romans
A Central Asian tribe is always plausible like Scythia, or the Huns, but the game already has you having nomadic starts and would make less sense going back to a more nomadic playstyle.
Goths certainly could be plausible too.
I suspect that a lot depends on how many 'strings' of Factions they want to have from Era to Era. IF they want to give a gamer the option of doing 'semi-historical' progressions, then there will be more specific titles for the Factions.
For example, Classical to later 'strings' might be: (-/- means there are two equally good choices for that Era)
(Persian) Achaemenid - Samanid - Safavid - Qajar - Pahlavi/Islamic Republic
('French') Gauls - Franks - Bourbon France - Imperial France - Republican France
('Greek') Greeks - Byzantines
(Chinese) Han - Tang/Song - Ming - Qing/Manchu - Chinese Republic/PRC
(Indian) Mauryan - Chola - Mughul - Maratha - India
('Roman') Romans - Carolingians - Holy Roman Empire
Obviously, some 'strings' reach their ends before the game reaches the end of Eras!
The fact that they have a 'nomadic start' gives me hope that they will somehow segue that into a possibility of Staying Nomad in some way. Another point here is that leaving out Central Asia is a really bad option: see Beckwith's
Empires of the Silk Road for a thorough discussion of just how important the factions in this area were in their influence on the surrounding 'settled' states like China, India and Rome and their importance in transmitting trade goods, wealth, technical advances, etc. Trying to do this all with 'Barbarians' or minor states is a poor substitute. There are also a host of potential Factions, starting in the Classical:
Scythians/Huns/Goths - Seljuks/Pechenegs - Mongols
- and after that they pretty much have to settle down or get swamped by the gunpowder-using settled states, as happened to the Mongol-successor Great Horde and the Siberian Khanates in Russia. I've posted before: this is the biggest thing in History that Civ leaves out completely, and I am still hoping that
Humankind will find a way to include it in some way.