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Some dumb idea I've been thinking in the last few days.
The scenario in particular takes an Abbasid Revolution that is suppressed by the ruling Umayyad Caliphate but the revolt cause enough noise that the Caliphate eastern provinces in great unrest as the multiple anti-Umayyad partisans are now leaderless but not totally destroyed.
So, could in this scenario the Dabuyid dynasty in Tabarestan exploit this turmoil in the Caliphate (by the time of Abbasid Revolution the Umayyads were either fighting or pacifying rebel regions though all of its territory) and launch a revolt that evolves in a full-blown conquest of Iran and Khorasan? Could this go as far as conquering Iraq and Jazira (kinda stretchy but eh)?