There is also the Tver Merchant Republic.
Also wonder, how/if serfdom would develope in such a scenario
Tver was a regular Russian principality, that is, a monarchy (maybe less autocratic than Moscow, but then, Moscow of the 15th century was not really an autocratic regime either - both Tver and Moscow had powerful princes
and powerful nobility, whose power would only be broken by the Muscovite occupation in Tver (1485) and by Ivan the Terrible's terror in Moscow (1560s).)
Medieval Russia had three republics - Novgorod, Pskov and Vyatka. All of them were noble republics rather than merchant republics (even though all three had lively trade relations with their neighbors, they dealt mostly in agricultural/hunting products, and this business was controlled by landowners).
Interestingly, there was no serfdom and no wish to introduce it in Novgorod (the greatest and best researched of the three republics). The great lords (some 40 of them) owned most of the farmland, and they did not fear peasants moving out of their estates to another lord's lands, because conditions in most of these estates were more or less equally bad/good (the lords did not try to outcompete each other with better conditions for their tenants, so acting as an oligopoly of sorts). There also was no free land to settle in core Novgorod areas (so-called 'five fifths of Novgorod land'), as all land there, even still unploughed, was already claimed by some lord. Therefore, peasants had to make do with their lord's exactions, or move out of Novgorod land altogether, and most stayed put.
As was pointed out here by previous posters, Novgorod was well integrated into the Baltic trade area and felt no need to subdue other Russian states, except Pskov, which they saw as part of their rightful domain, and tried to reconquer a few times. Of course, had they known that one of these other Russian states would grow strong enough to subdue Novgorod itself, with disastrous consequences for every Novgorodian lord, they might have rethought their non-intervention stance.
However, even had Novgorod united Russia, it would likely be forced to transform itself into a highly militarized and authoritarian state to conquer, rule and defend their empire. Moscow's example is instructive, as the ruling house's powers there grew in step with the state's military commitments, and the subjects' freedoms were being curtailed accordingly, resulting eventually in autocracy, serfdom and lack of ecclesiastical autonomy.