[Saulot]

It seems to me that Parthia is somewhat well-suited for an amalgamation empire with 4 particular disparate groups. However, I'm not sure that it really fits into the Late Era, and I'm not sure that it fits the degree of difference in the four groups. Perhaps very light inspiration would be the most fruitful route. (Do we really want to rely on light infantry & light cavalry archers?)

Since the general concept here would be a backdrop where trade/commerce or multiple military invasions / successive empires that have placed four seperate ethnic groups into one area; the Middle East could certainly be such a backdrop, as well as Iberia, Sicily, Central Asia, etc. (even the British Isles could be made to fit)

The guild concept is nice in general, but seems ill-suited to this situation. I feel like a better story is that these four groups are actually at odds with each other. They obviously have different magical practices, they should also speak different languages, follow different cultural practices, utilize different military traditions, and have before worshipped different deities. The mechanism of guilds in this case, just simply isn't strong enough.

However, they face great outside pressure due to their wealth from trade, and are united by a strong leader (the pretender) who uses personal charisma, magical force, a priesthood that is on the rise / increasingly popular, and subterfuge (an extensive secret police / spy system) to put the feuds on hold for the time being and able to form one city-state. (Though a confederation of states is an idea of some merit as well.)

Oh, and in reference to Zoroastrianism, while my knowledge of antiquity is a bit rusty, I believe that it certainly flourished to some degree during that time (as it had become popular some centuries earlier and was still around some centuries later), but that as many things Parthian, there is little record of it.

By the way, in regards to air, I haven't been completely idle (I've been doing some conceptualization work).