A Crippled Muscovy

In 1445 King Kniaz Rurovich of Tver dies without an heir, Muscovy (whom shares the same dynasty as Tver) inherits the throne and the two countries form a Personal Union. Lithuania still has some independence from Poland and thus attacks Muscovy over the throne of Tver.

The resulting war absolutely destroys Muscovy's army and morale. The Golden Horde and Kazan eventually invade too. The Lithuanian peace treaty with Moscow essentially breaks up the would-be-empire as Perm and Novgorod gain lands. In the south the Hordes gain land and prestige.

Lithuania eventually falls under Poland's control and thus retires from the diplomatic scene in Russia. The Islamic Hordes are now once again the dominant powers with enough manpower to overcome the more advanced Russians.
 
Boris of Tver was still alive, along with his two sons. Lithuania would need a good reason to attack Muscovy, their relationship fluctuated from time to time. The Hordes have squabbles among themselves and also had in some cases decent relations with Muscovy.

If you want to plausibly take down Muscovy you would have to do it rather early in the 1300's.
 
In 1445, Moscow is in the middle of a civil war between the Suzdal and the Zvenigorod branches (Vasily the Blind vs. Dmitriy Shemyaka) - there is no shortage of pretenders to the Grand Princely throne.

However, let's say Vasiliy the Blind dies - he is the man whose accession paved the way for direct rather than lateral succession, whose political activity eliminated many of the smaller principalities within the Grand Principality, and who declared the Russian church independent of Constantinople.

Shemyaka takes over. He represents the medieval Russian law, and he's deeply unpopular in Moscow itself. I can see him neglecting Moscow and strengthening other towns - Vologda or Zvenigorod, instead.

If he was the man who would reject the union with Rome as per Vasiliy above, perhaps there's a chance that Moscow's boyars would reject his rejection.

Religious matters are a good reason for Lithuania to intervene as well.

But in short it's hard to kill enough of the Moscow Rurikids to even clear the way for Boris of Tver, let alone someone outside the Vladimir branch of the Rurikids, and while Ulugh Mohammad was an energetic and successful general, Kazan by itself just didn't have the strength to take any of the major Russian cities in Zalesye at the time.
 
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