Sometimes, a monarch dying without male heirs was a problem back in the medieval age. Casimir III of Poland was one case: after he died without a male heir, the native polish Piast dynasty went extinct, and Poland ended up as ground for power plays between Bohemia, Hungary, and Lithuania.
So, what if Casimir had a son, sometime around the 1350's? What happens to Poland under a continuing Piast dynasty?
What will polish foreign policy look like? Will they still ally with Lithuania against the Teutonic Knights? How will a continuing Piast Poland interact with Bohemia and Hungary?
What about domestic policy, particularly towards the nobles?