Zioneer,
why not Mazdakism? If you want a grassroots Zoroastrianism, there`s nothing better than Mazdakism. A successful social revolutionary movement is what could have found ample resonance in the Chinese and Arab world, too, at the time. (Islam was one, too.)
ImmortalImpi,
dangerously extremist? On the contrary, I would say that all the autocracies of 500 CE were dangerously extremist. Look at the mess they made, warring with each other, devolving the functions of the classical state to petty aristocratic tyrants, squandering the infrastructural, technological and philosophical state of development, being unable to withstand the onslaught of nomads.
Mazdakists could either have overthrown the rule of the Sassanids, or merely strengthened its more progressive-minded members like Kavadh. Either way, by pressing for a more egalitarian outlook, they might have broadened the foundation of support for the state of Eranshahr. Around 500, the time for a more political world religion was ripe. Centuries of otherworldly redemptorist sects had preceded it, and lost their innocence with the installation of Christianity as the state religion in Rome/Byzantium, Aksum, Armenia, without bringing the "community of saints" any nearer.