With a POD no later than 1420 but no earlier than 1395 AD, let Novgorod stay independent and not become absorbed by the other Russian principalities. Try to do it as plausibly as possible, take the economic vulnerabilities into account (Novgorod needing to import quite a lot of grain, mainly from the more southern Russias), but let Novgorod survive in such a shape that it can keep independence or re-establish it after a few (say, 10) years of domnation by another neighbouring country.