My Scythia game at the moment:
I'm at 264 B.C. Everything is going ok so far. Expansion of the other factions looks to be historical, I was actually surprised that Egypt wasn't quickly eating up the Seleucids and instead had lost some territory in the Israel area to the Seleucid Empire. I find it a little odd that they have the city of Kasia at game start as it is far separated from the rest of the Scythian territories. My game CTD'd right after taking Olbia, when loading from the battle map back to the campaign map. That is probably a BI bug though. I'll finish up and edit this post later.
Edit: I have 20 turns in, I'm noticing that no one is attacking me. There were opportunities where the Greek City States and Parthia could have easily besieged me as I had my garrisons thinned out, but they did not. I might play a little more to see what happens. Other than that the factions are expanding nicely. Egypt, just barely being able to regain a territory or two from the Seleucids expanded down into the Arabian peninsula (you might want to make Arabia an unconquerable territory since it's just a large desert expanse.) It also seems to me that the Scythian horse archers might be a little over powered. Someone else may need to test that to confirm if they are.
Suggestions:
Maybe add traits for barbarian factions that would somehow represent how they were a collection of various tribes and not one centralized country. I believe RS and EB do something like this? This could probably also work for the factions like the Greek Cities and Hellenic Cities.
Get rid of the dogs and pigs of course.
Maybe for the city maps you could get a hold of RS devs and see if they will let you use those? I have always found it a bit painful that in RTW all the cities have massive mega buildings, massive column monuments dotting the city etc. Hegemonia has great city maps for the Greeks as well.
Parthia:
I was only able to play to 265 B.C. Parthia starts out with a large horse army with almost no infantry and hardly and buildings in their settlements. Debt starts building quickly unless I pretty much disband my entire army and leave myself vulnerable to the wandering Seleucid armies. I was able to take one Seleucid settlement and the Seleucids started spamming militia hoplites at their neighboring settlements making it impossible to take. This faction will be hard to balance, on one hand they need at least a barracks in their capital to recruit basic infantry. On the other hand giving them to much leverage could overpower them as they have really powerful cavalry and start out with numerous horse archers and two heavy horse units.
Other suggestions, Parthian slingers need a better skin, an empire would have to be incredibly insane or insanely rich to outfit peasants with slings in purple robes.