This is, strictly speaking, a butterfly of my tl that I'm considering expanding on. I'll spare the details but suffice to say the timurid empire seems impossible at this point and the ilkhanate collapse earlier. Meaning the golden horde doesn't get smacked around and weakened for the rus to take over.
It was already on a decline before Timur’s appearance on the scene. There was a protracted period of the internal instability and it was split into the White (Siberian) and Blue (Volga) Hordes. Totkhamysh managed to unify them
with Timur’s backing but the result was not stable.
In the optimistic scenario we may assume that the GH evolves in a reasonably stable semi-sedentary state living to a great degree off the trade routes. There can be numerous scenarios of it’s further relations with the Russian princedom(s) but it worth noticing that by the time of Totkhamysh it started lagging behind its main western opponents technologically: both Princedom of Moscow and Lithuania already had (rather primitive) artillery. It is also anybody’s guess for how long the GH would be able to maintain its territorial integrity and internal order.
In that scenario, could the GH eventually feudalize more and wind up triumphant over the rissian states? If so, what might this entail for islam and eurasia?
The GH would be very lucky to survive in a long run as a single entity. It could not “feudalize more” (whatever this is supposed to mean) because as long as it was a single state, it’s ruler was an official and mutually recognized feudal overlord of the Russian territories, except those which belonged to Lithuania.
It’s continued existence, just as the OTL scenario, would do nothing to impact status of Islam in Eurasia: the GH did not try to convert the Russian territories into Islam and the later Russian conquests did not involve a forced Tatars’ conversion into Christianity.