I'm surprised having a Moslem Russia hasn't been discussed more. It was definitely possible and would have been literally huge.
Two possible PODs are having the leaders of the Rus convert to Islam instead of Christianity, which at least according to legend they considered doing, or the POD discussed in another recent thread of the Volga Bulgars being the Mongols' enforcers instead of Muscovy. A super strong Golden Horde might also work.