Allright, here's my try for this scenario:
The Umayyads manage to survive until about 900 AD (albeit with some trouble), and continue to practice their policy of tax-farming the non-Muslim population and preventing large numbers of non-Muslims from converting to Islam.
(in OTL, there was only one Umayyad Caliph who promoted Islam among his subjects and encouraged them to convert, and this eventually led to the downfall of the Umayyads as his successors were not willing to give the fairly large group of converts equal rights and tax-exemption, which resulted in various revolts from dissatisfied converts)
Then, by the beginning of the 10th century, the Umayyad state is weakened by a few internal rebellions, and it fails to respond properly to various Turkish hordes who are now migrating into Persia and the Middle East. And as the Samanids are butterflied away in this scenario, Islam has far less influence in Central Asia ITTL, so the lionshare of these Turks are not Muslims.
Then, a major pagan Turkish tribe conquers the emirate of Arminiya, one of the provinces of the Caliphate, and several Armenian nobles side with the Turks in the hope of gaining more power.
The Armenian nobles intermarry with the ruling clans of this Turkish tribe, and both clerics of the Armenian Orthodox Church as well as pious Armenian aristocrats promote the Armenian Orthodox version of Christianity among their Turkish overlords, and after several decades, the ruling clans of this Turkish tribe have become largely Christian as a result of proselytization and intermarriage with Armenians.
Eventually, this results in the Armenianization of the ruling elite of this Turkish tribe, and even though most of the nomadic Turks retain their Turkish identity and language, the empire itself becomes decidedly Armenian in character.
This Armeno-Turkish empire then also has a few powerful and capable leaders, and it conquers both Azerbaijan, as well as parts of the Eastern Caucasus, Byzantine Armenia, and some parts of northwestern Mesopotamia.
Persia has been overrun by various Turkish tribes at this point, and the Umayyad Caliphate is losing its grip on Mesopotamia, where both invading Turks, individual Arab and Kurdish tribes, and Islamic sects and movements that opposed the Umayyads are now taking advantage of the near-collapse of the Umayyads. The northern parts of Syria and Mesopotamia are conquered by the Armeno-Turkish empire.
And on the north, this newly established Turkish empire is bordered by the emirate of Tbilisi and various small Christian Georgian principalities, all of whom are vassals of this Armeno-Turkish empire.
In the west, the Byzantine Empire remains fairly strong, but aside from a few minor conflicts, there are no real wars beteen the Byzantines and the Armeno-Turkish empire during this period.
During the 10th and 11th centuries, the Armeno-Turkish empire remains strong, while the Umayyad Caliphate, which is now reduced to Syria, Palestine, the Hijaz, and Lower Egypt, deteriorates even further, and eventually collapses during the mid-11th century.
Syria and Palestine are conquered by the Armeno-Turkish empire, which makes this empire the major power of the Middle East.