That rules out mass conversion to Islam. The Arabs could be temporary masters, such as the Mongols in China, not making enough converts, such as in Spain (ops, al-Andalus), to be expelled after no more than a century - say, the Abassid revolution is a Zoroastrian one. Then the (heathen) Seljuks fragment the subsequent state and gain ascendacy, short of total domination, before heading further west (they do? if not, this would be a major problem). Khorezm could be a Persianate society gaining the upper hand throughout the Persosphere... till the Mongols come out of Hell and raze anything to the stone age as they did. Persianize the Mongols, and you'll have later a nativist Tamerlane, a ferocious enemy of the Persianized Ilkhans and of all settled civilization (oh my gosh...). Then, provided you find an equivalent of the Safavid revolution baesed on some Zoroastrian sect, you could find later Persia harassed by Hindu-heathen Afghans, and later reunited and brought to its maximum power by a Nadir Shah capable of unifying everything frm the Euphrates to Delhi, from the Aral Sea to Oman, from Sindh to the Caucasus. Another Cyrus, in other words. And then a decline with internecine wars, one or two dynasty changes, modernization, a superstate taking part in both world wars and later in painful "inner decolonization" conflicts from the Caucasus (Azeris, Armenians...) to Central Asia and Afghanistan, to the point of collapsing and shrinking dramatically.