Well, all empires fall in time, but the Sassanids, like the Chinese OTL, might well end up something of a rise-and-fall "eternal empire", too prestigious not to revive, at least in its Iranian core.
I've had similar thoughts. The legacy of the Sassanid Iran will have a very long historical reach, much like Rome. Picking up the mantle of the Shahanshah will something many people aspire to.
Constantinople, that's a pretty cool scenario. Looks like most of OTL Russia might end up Turkish in this TL.
Coming from you, that means a lot!
It's taken awhile to get around to doing it, but totally worth it.
And yes, *Russia will be largely Turkish, and be the center of more than one significant power. Depending on how things work out, it'd be fun to see the rogue northern Slavs and Golindians survive too.
One quibble: you talk of the Saxons being pushed "east" when you clearly mean "west."
Oh shoot.
I wonder if under the influence of tolerant Sassanid rule the different branches of eastern Christianity will come to be be seen as just different "schools" like the various flavors of Sunni Islam OTL, or whether they'll start slaughtering eachother again if the Sassanids drop the ball.
That's definitely my plan, at least in the Near East, Africa and *Europe. Miaphysitism, Chalcedonianism, the orthodox Nestorians and to some extent the *Catholics will probably never be united organizationally, but, like you said, it will be more a competition between *generally* cordial schools of one religion.
In Northern *Europe, *Turkey, the Steppe and China... it will be a different story.
Christianity (in as much as we can even give it one label) is going to be both very different and significantly more widespread in the future. Europe is far from the center of Christianity, and with out that centralism, the religion is taking some rather different turns. Without Islam coming in and messing up Asian Christianities fun, it's likely that in a few hundred years, you could take a *Catholic from Burgundy, pair him up with a Celtic Christian and they'd think of each other as misguided but generally right. If you then took both of them and put them in *Turkey, they'd recognize the Christians there as Christian, but no better than heretics. If you took them and put all three in China... well all three would turn to each other, ask for forgiveness for judging them and in turn denounce the Chinese Christigristasrian as no better than a pagan.
Christianity will certainly be much more of a spectrum than it is in our timeline.
Will those Tang remnants be able to unify and hold off the Turks, or does it seem likely that the Gong will unify all of China? IIRC, the Turks of this era haven't quite as an effective a military edge as the Mongols a few centuries later, and it took them a long time to finish off the south...
The collapse of the Tang before the Turkish invasion was pretty thorough, so it doesn't look like they'll be uniting any time soon. Whether or not they are conquered is another story. Once Gōng control is consolidated and get their roots in, it shouldn't be too difficult.
Nice to see the Tibetan empire get some love, but its probably doomed either to end up wagged by one of its tails or break up: the Tibetans are demographically just too small a people to maintain rule over that large a territory in the long run without considerably widening the definition of "Tibetan." What's going on in Tibet religiously?
A Tibetan Empire is just too cool to pass up... but yes, it certainly can't last as it is. The recent vacuum in China and the generally divided nature of norther India has helped their longevity for now. Religiously, they're not far off where they were in OTL, but there is a rather higher number of Nestorian monks (as happened in OTL) running around, as well as a number of Manichean and Nestorian merchants and traders.
Glad you liked it! If you have anymore questions, please let me know.