The republic of Novgorod was the biggest Russian state by size, although not by population around the PoD. 1227, Yaroslav of Novgorod attacked Finland as in OTL. Without the Mongol attacks on Russia, neither the Teutonic Order nor Sweden dared to attack Russia, so there never was an Aleksandr Nevsky. Novgorod continued to reign in the North and slowly started to expand East. 1270, a rebellion of peasants against the rich boyars took place in Novgorod. The German explorer Markus Poller came to Novgorod in 1282.
In the 1320s, the Novgorodians would start to explore Novorossiya (New Russia), OTL Siberia, starting with the Ob river. When the Russian princedom of Polozk was conquered by the Teutonic Order 1322, parts of the population fled via Smolensk to Novgorod, after Smolensk was unwilling or unable to help them. City names in NE Russia like Novopolozk and Nishny Polozk would tell about their wanderings.
The downfall[]
1397-1401, Novgorod was hit by the Black Death.
Vladimir-Suzdal wanted to solve the Russian Schism by war (1413-22), but Kiev and Chernigov allied with Novgorod and Smolensk against it, and managed to win. This coalition fell apart soon after, however.
1468, a delegation of merchants from Vladimir sent a delegation to the Grand Prince, asking him for help against the competing merchants from Novgorod. This lead to the War between Vladimir and Novgorod 1469-72. Novgorod lost the right to trade in the areas east of Volga and Kama - thus being cut off from Novorossiya now.
Since the 1480s, the "Judaizing" (Jews converted to Christianity now preaching their teachings) started to appear in Novgorod too.
In the Novgorodian War 1512-17 Vladimir-Suzdal defeated the old competitor, annexed it. Many Novgorodians who missed their old freedoms went to Novorossiya.