There is no getting around it, we're going to need someone from CA to answer one very short, very simple question, in order to add new factions:

Where, or how, do we define which culture (Roman, Barbarian, Carthaginian, Greek, Egyptian, Eastern) should be assigned for each faction?

According to Jerome Grasdyke, here, we should be able to add new factions:

https://forums.totalwar.org/vb/showthread.php?t=37359

We have been searching long and hard for how to do so, and really right now we come down to what seems to be only one small obstacle. How to tell the system to use a particular culture for a particular faction. It would be nice to define a new culture, but we can cross that bridge when we get to it. We can use existing cultures if we have to, just to get the ability to add more factions.

We have found no area where this might even be implicit - from what we can tell, this seems to be hard coded. Unless CA pops in and tells us where to do this, I don't think anyone is ever going to be able to move beyond the number of factions we have today.

Sorry, guys.

-khel