The emergence of Moscow was heavily tied to the Mongol invasions, since the Mongols destroyed a lot of other power centers in Rus and eventually gave Moscow the responsibility for collecting all the taxes in Rus (allowing Moscow to become much more wealthy and powerful). So if you somehow avert the Mongol invasion Moscow would likely not rise to prominence. This doesn't totally clear the way for Novgorod to remain the strongest state (there are other principalities they have to deal with, most notably the Grand Duchy of Vladimir and the Kingdom of Galicia-Volhinya), but it does eliminate their main rival IOTL. From there if Novgorod invested more money in their military and were able to manage the worst aspects of medieval democracy they would be in a really strong position going forward. However, a Novgorod-led Rus is unlikely to be the strong, centralized state that emerged under Muscovy. Long-term you probably end up with a decentralized confederation of principalities with Novogord at the center (or in other words Rus would be similar to Poland-Lithuania).