Due to the large Iranian population, I reckon that any Monotheistic development may be influenced by Zoroastrianism. With some of its adherents probably resentful of the past Macedonian conquest. A possible future scenario might be that a new Iranian dynasty were to rise to power in the wake of the Argead collapse, they might ally themselves with a powerful underground movement of Persian zealots whom yearn for the past glories of the Achaemenid era.
If the ruling powers maintain Polytheism, they could be in a position to re-mold, regulate, and centralize all religion within the Empire without offending the piety of the subject provincials. Create a new full-time clerical structure that unites all or most of the regional cults. Religious officers or "Episkopoi" (Overseers/Bishops) could be appointed by the Basileus to administrate over the various priesthoods within the Satrapy, perhaps.
And this may not be be too important to the overall picture, but another, if often overlooked, hotbed of Monotheism would be the Getae tribes that lived south of the Carpathian mountains. In the First Century BCE, Burebista, a contemporary of Julius Caesar, united the Getae/Dacian tribes, conquered swathes of land throughout the Balkans, and was a follower the mysterious Healer-God Zalmoxis, even going as far to reset his people's calender to the year of Zalmoxis' alleged birth in the 700's BCE. Just thought I'd put that last one out there.