Hey. I've been doing rearrangement for vanilla map and I'd like to share my view on the whole Persia thing.
Geographically (and as such historically) there're 6 distinct regions east and north Zagros. They are:
* Azerbaijan - the grazing ground of Persia, including Shirvan and all the way to the Caspian steppe through Derbent. This is where Seljuks settled en masse, this is where Mongols had their first capitals (precisely for the reason of lovely pastures), and this is where from all those nomads were projecting their power on Armenia. If
this image is still valid, I suggest splitting duchy of Azerbaijan into Azerbaijan (centered on Tabriz) and Shirvan. Look up Shirvanshahs - a very peculiar duchy and dynasty. A reason to stretch the kingdom towards Pontic-Caspian steppe is to give it border with the Golden Horde. I'm sure you've read about wars over Azerbaijan - again, precisely because of how awesome the pastures here were.
* Jibal, otherwise known as Persian Iraq or Iraq-i Ajam, including Lorestan, Rayy, Hamadan, Esfahan.
* Codename Fars - an area encompassing Shiraz, Yazd, Kerman. It's geographical justification is that it encompasses southern extension of Zagros mountains. Here's a very nice map
As you can see, these two areas are separated from eastern Persia by two huge deserts: Lut and Kavir.
* To north-east, between Kavir and Khwarasm/Transoxania/Mawarannahr lies Greater Khorasan, stretching all the way to Balkh. It's another region favored by nomads, Oguzes put particularly strong pressure on Great Seljuks here.
* To its south lie Afghanistan and Indus border. Depending on the treatment map extension gets, it could be two more kingdoms (Afghanistan and that Sindh thing including Mekran). Edit: Actually, having taken a look at dev diaries screenshots, everything seems setup for these three kingdoms:
* And then there's inaccessible Tabaristan that includes Daylam. A very small realm, but due to it being so inaccessible, it managed to remain virtually independent for large periods of time. It took major efforts both by Ilkhans and Safavids to subdue it.
Finally, to the farthest north-east lie Khoresm/Khwarazm (south/south-west from Aral) and Transoxiana/Marawannahr (behind Oxus). Why Paradox sticks to the name Khiva is beyond me. Whether Khoresm deserves being a separate kingdom or be merged with Transoxania is a matter of personal preference and balance I guess.
This is how it looked in the end superimposed on vanilla map