All great ideas, though I doubt we will see them.
I'd be intrigued to have a ancient era nomadic civ, though that might be too unbalanced? But it would seem kinda fitting, continuing to be migratory.
An Ancient Era 'pastoral' or migratory Faction would be either Fantasy or Tricky. I was plugging for this, too, until I finished reading a couple of academic books on the recent archeology of Horse Domestication in the past year. Now I can see the problems, and I suspect I know why
Humankind doesn't have a Hun-Mongol-type Faction in the Ancient Era since they also have an in-house Historian.
They have now tracked 'bit wear' - the physical changes to a horse's teeth caused by wear on a cloth, rope, leather, or metal bit between his teeth when he/she is being ridden or driven - back to 3700 BCE: solidly Ancient Era. Problem is, there is no evidence of horses being used in combat for another 2500 - 2800 years! The problem was not with the ridable horse, but with the weaponry. The only bows were self bows that were too long to be easily used on horseback, or stone/copper/bronze maces that required you to swing down and out at someone from horseback and therefore had a much higher chance of pulling you out of your primitive saddle than, say, a straight-ahead thrust with a spear or lance would have. You could throw javelins, but that gave you no advantage over the man on foot throwing javelins, and if he hit your horse (a much larger and easier target than he was) you were on the ground, possibly pinned under your horse, and Not Long For This World.
It was not until about 1000 - 800 BCE that the composite laminated bow reached the steppe (there are examples of primitive laminated bows from Tut's tomb from about 300 years earlier, but this may be Independent Development: Egypt was and is sadly lacking in decent bow wood, so had a solid reason to develop an Alternative) and right after that, you have the Cimmerians raiding into Anatolia and the Scythians a few hundred years later giving the Anatolian/Mesopotamian States a hard time with mounted archery.
But that is at the beginning of the Classical Era.
So, technologically, your classic Steppe Nomad Horse Archer is not an ancient unit at all, and so one of the prime reasons for having a pastoral/nomad Faction has no historical basis for the Ancient Era of the game.
It could still be done. Once they started riding horses, the first major effect on human populations was that the herding groups (Sredny-Stog, Kura-Araxes, Yamnaya Cultures, for examples) could now move herds and flocks onto the 'high' steppe, away from the rivers, because the herders were more mobile and efficient (a man on horseback can successfully 'herd' and protect up to 4 times more animals than a man on foot), which allowed them to 'colonize' the real steppes. It also meant that instead of permanent camps in the river valleys, they could use horses to move the entire population, following the herds, becoming truely mobile/pastoral for the first time.
So, there could be a 'nomadic' Faction for the Ancient Era, but the characteristics would be some kind of Outpost Only settlement, and Tech heavily weighted in certain directions: historically, there is strong evidence that the 'pastorals' invented effective saddles and tack for riding, spoke-wheeled chariots, (independently) the composite bow and lost-wax casting of copper and bronze (several horse-using cultures like the Maikop were very, very proficient metal-workers), so some mechanic to allow them to progress technically while not building cities would have to be introduced.
It's pretty late for that in the development cycle, but would certainly be a possibility for a First Expansion.
; but I'm familiar enough with the game to not care more about it.
