The Novgorod Republic existed in northern Russia between 1136 and 1478. It was the easternmost point in the Hanseatic League, the north-western terminal of the Silk Road, and by all accounts immensely prosperous. It was a democracy with a very permissive suffrage (even lower classes could vote, though serfs could not) and high level of literacy.
Novgorod was too far north to suffer militarily from the Mongols, but it didn't help them much:
- They still had to pay tribute to the Mongols
- They were a target for Catholic crusaders from Sweden and the Baltics
- They were engaged in wars with the other Russian principalities, most importantly Muscovy (Moscow).
Novgorod had some success playing its enemies against one another, but Muscovy eventually grew too powerful. When Novgorod tried to ally with Poland-Lithuania, Moscow's ruler Ivan III declared war, defeated Novgorod's army, and crushed any hope of independence.
What is the smallest change that could avoid this outcome for as long as possible, without simply supplicating Novgorod to another overlord (Sweden, Poland-Lithuania, etc)?