All right, today @ work I had a moment of inspiration for the long-term future of the TL. Here goes...
1) We pull off Dominus's Crusade against the Vikings. This'll be a pretty big deal in and of itself. If you want to get it going soon, perhaps some Vikings aid their co-culturalist Saxons against Arthur and friends and that leads to bad blood that gets REALLY bad when the real "Viking Age" begins.
2) Lots of Viking refugees go down the Volga and Danube, setting up various statelets somewhat resembling OTL's. The beginnings of the Russian state. I might be able to swing some of these, as Russia is my responsibility.
3) These grow into the Russian states of OTL. If the Crusaders are Western Christians (probably...hard-core anti-injustice stuff of the Visigothic-Catholics), the Russians could adopt Orthodoxy, or perhaps they remain disorganized-Norse-pagan (all the easier to convert them to Nestorianism).
4) The Mongols arrive and conquer these states. The Mongol elites largely convert to Nestorian Christianity, while Islam becomes popular among the commoners. Increased contacts between the Nestorians in the East and the Western Christians lead the West to acknowledge the Nestorians as fellow Christians.
5) Tamerlane (or someone like him) is brought to power in a popular revolution. He begins persecuting the ex-elite and common Nestorians, with full-on genocide the ultimate goal.
6) The Western Christians launch a new Crusade against Tamerlane for his murder and mutilation of fellow Christians. This puts an end to the shenanigans, at least temporarily.
7) A Central Asian Saladin arises and defeats the Crusaders. He's very anti-Nestorian (he views them as collaborators) and fearing a renewal of the vicious persecutions, they get the heck out of dodge and head for Russia, where perhaps some elements of the Nestorian Mongols still rule (something analogous to the Golden Horde).
8) Due to increased intermarriage with the populace (no religious difference; the GH was Muslim, while TTL Nestorian Mongols are Christians), there's some cultural mixing. We have
(drumroll please)
The Russian Khanate; Ruler is the Khan of All Russias. Official religion is Nestorian Christianity; unsure whether to have church-state separation or not.
9) Delayed discovery of North America, as the Crusade has kept the Eastern trade routes open for awhile longer than OTL.
Do y'all like this? Some have said that very long-term future planning is a bad idea, so if y'all have caveats or want a totally different future, feel free to let 'em rip.