Interesting. Would be cool to do a TL where they go for Egypt and also defeat the Byzantines in a Manzikert-style battle. In such a TL, could we eventually see Western Europe engage in Crusades against the Tengrist Gokturks to help the Byzantines and also liberate the Holy Land from heretics and infidels?
Maybe, though I couldn't predict things that far out from the POD nor whether such a crusade/crusades might be ultimately successful. Worth noting that the Gokturks might not remain Tengrist forever, them converting to some other religion (Nestorian Christianity perhaps, or Buddhism, or even Manichaeism as the Uighurs did for a while - as
@History Learner said Zoroastrianism seems to have been on the way out this late into Sassanid times) is possible as well.
You actually touched almost exactly on an idea I had the other day.
Gist of it was the ERE seeks a compromise with the Copts/Syriacs, but provokes an earlier split with the West that allows more Western varieties to flourish because of the weakened state of ERE influence and the Papacy still coming into its own at this time. Donatism in North Africa comes to unite the Berbers of North Africa-they appeared to be having a heyday of sorts between the 600s to 800s-and they take advantage of the traditional Visigothic infighting to conquer much of Iberia. In effect, you still get an Al-Andalus analogue, but it's Donatist and firmly Berbe dominated fighting the local Catholics still.
I think the Donatists were a spent force by the 7th century - their heyday was in the 4th and 5th, but by the 600s they had been more or less broken by successive Roman (peaking with Saint Augustine of Hippo's efforts to combat their influence), Vandal and then Roman (again) persecutions, with whatever remnants of Donatism that managed to endure in the remote corners of Roman Africa being swallowed up by Islam afterward historically. Of course, without Islam around I'm loath to suggest that a late-game Donatist comeback is
impossible, but it does seem unlikely to me.
Worth noting that some Romanized Berber monarchs did help the Byzantines in Carthage fight off the rising tide of Islam for some time:
Kusaila and
Dihya were both pretty strong leaders who seem to have held out for many years and given the Rashidun armies a good fight IRL. However, although they were Christian they don't seem to have been Donatists - they had good working relationships with the Byzantines and I've never seen any source note them to be of a Donatist persuasion, which I'm sure some Byzantines would've made note of if they had been forced into working with 'heretics'. Rather, I think the Mauro-Romans
would've more or less been aligned with Rome in any conflict with Constantinople (drawing their theology from the mind of Cyprian, Augustine, etc. rather than the defeated Donatus Magnus, and further indicated by their support for the anti-Monothelite Gregory against Heraclius' Monothelite grandson Constans II). Kusaila (for example, you can substitute another Berber monarch or ambitious Mauro-Roman patrician like Gregory here) trying to get the Pope to crown him Caecilius I of a restored Western Roman Empire, and using that as his excuse to both kick the Visigoths' door down and to push into Italy, seems to me the likeliest cause for a collapse of Byzantine temporal & ecclesiastical authority in the west.
As an aside, no Islam means
the African Romance culture & language which dominated in Roman Africa's coastal cities won't be wiped out. A Christian Berber king or imperial usurper who sets his capital at Carthage (IMO the most logical place for it) will probably embrace that Romance culture himself, rather than impose the opposite and try to Berberize his wealthier & more populous urban subjects. Overall I think most of the Maghreb ITL will likely resemble an extension of southern Europe, and grow culturally closer to Spain and Italy (where their most obvious trading partners & allies against the Byzantines are located) than say, the Saharan Tuaregs - African Romance was identified as being close to Sardinian by Arab chroniclers, and apparently underwent betacism like Spanish did.
This I'm a little more iffy on, if only because I think the Empire can still capitalize on weakness of its neighbors, especially given a couple of generations to recover and work out the kinks with the Eastern Christians within it. For example, if the Nestorian/Zoroastrian divide really does shake up the Sassanids, taking Mesopotamia and establishing the new border on the Zagros seems possible. With the Persians finished as a major threat for a time, the focus can shift back to the West.
As with a Donatist comeback, I'd prefer to never say never re: the Byzantines managing to carve more bits off their rivals in moments of weakness. But I do think it wouldn't be easy, hence the difficulties in shifting the Roman-Persian border either way in most of their past wars. Again, not ruling it out (I actually have the ERE expanding eastward in one of my own timelines) but it'd require a good bit of luck on their part and great misfortune on that of the Sassanids, who though beset with extremely bad luck in the mid-600s IOTL, may or may not have gotten over it by the time the Romans too have recovered from Khosrow's war.
Likewise in the west, where the Visigoths have already driven the Byzantines out of Hispania (for good, at least historically) in the late 6th century and the Lombards really did a number on their Italian possessions around the same time. IMHO these setbacks have shown the real limits to Byzantine expansion westward and would be tough for the empire to overcome (beset as it was with plagues, Avar/Slavic incursions and the Persian war), at least within a timeframe that I could still reliably make predictions of without just creating a full-on TL.
Most of my interactions with Featherston were on the ACW and I was unequally impressed, for whatever that's worth when discussing his view on this; as you note succinctly, the actual evidence supports quite the opposite outlook. Indeed, I don't see how one can have this conclusion of his when the ERE not only managed to beat back the Persians in their Final War, but then go on to survive the Arab conquests while the Sassanids didn't.
What's your opinion on the viability of Mazdakism? I've seen it argued it could've been the force to reinvigorate the Zoroastrian religion at large, but I'm doubtful.
I don't think I was even around (at least as more than just a Guest) before that fellow got banned, but such weird statements and his rather combative way of phrasing them gives me the impression of someone who gets his jollies out of being a contrarian.
Anyway, I'm also skeptical of Mazdakism being able to revive Zoroastrian fervor. Seems to have had too much opposition to its doctrines among the Persian elite, who were very much running the show in the empire by the time of the unfortunate Yazdgerd III. Mazdakism's strong emphasis on egalitarianism & common ownership lends to it a revolutionary character, hence why its successors (like the Khurramites) were often in rebellion against the established powers in Persia - I don't really see them making a comeback since Kavadh I got burned after playing with Mazdak, unless a Sassanid civil war or two (such as the one plaguing Yazdgerd's reign after Heraclius' final victory, for example) turn downright apocalyptic for Persia and create the opening for a Mazdakite leader to attract a huge following against the warring nobility.