I'm not sure if this has come up before, but was the ideal of nationalism- as in "nation state" of common culture existing under a single government- inevitable?
Nationalism OTL heavily depended on education and modernization; the growth of trade and commerce and the intelligenstia provided a pressure for cultural awareness/cohesion across what had previously (and in many ways remained, and remains to this day) a heavily fragmented and regionalist Europe. That said the example of Switzerland demonstrates that culture can transcend religious and ethnic ties to some extent.
In an ATL where, say, the Roman Empire (of eastern or western variety) remained intact a la China and its dynasty system, would the eventual rise of literacy serve to create regional "national" identities, or is it possible to foresee a large, sprawling empire? If Poland-Lithuania did better could it survive by incorporating different ethnic groups, or are such tensions inevitably lead to strife and collapse?