I would focus on the East. The Abbasid Caliphate fought the Tibetan Empire around 810-815, invading from Kashmir but were held off by the forces of Tride Songstsan. What if they were not held off. We could have a Muslim instead of a Buddhist Tibet. (Of course if the Tibetand had held Kabul from the Abbasids, which is how the war started, we might have a Buddhist Afghanistan today

). Perhaps the Caliphate then defeats the Uighur Khaghanate, taking over Mongolia (or alternatively, the Abbasids just defeat the Uighur Khaganate, which is Mianichean leading to a Muslim Mongolia and a later attempt at Islamicizing China in the 13th Century under the now Muslim Mongols).
Lots of possibilities here.
If the Tibetans and the Uighurs are both conquered by the Calipahte, I would look to Tibetan tribes and Buddhist lamas migrating as refugees to the last possible refuge, Northern Siberia, where they might expand beyond the reach and notice of the Caliphate. Since altitude is equivalent to latitude, the Tibetans with their barley and yaks would be well adapted to surviving in the Lena Valley and expanding from there.