I'm of the oppinion that it's helpful to have a certain level of freedom for NRPs. So I'd lean towards it being a Sandbox. That way hopefully the writers have the chance to try on their shoes and make it fit before going out if they haven't already. But then for those who might have trouble getting started it might help then for there to be something central. I hesitate to ever describe this as 'the plot' because that sounds too rigid, but I'd call it a concept for the people involved to associate with or orbit around as they write.

Failing that, then I would say it should be the responsibility of the GM to act as this central premise his or herself when a central story pretense is absent to act as the anchor that brings players together to interaction. Whether or not that's the nation that's the most politically central to the world, the strongest, or the one on the warpath (or diplomatic or economic warpath) to whip up points of conflict. The key thing in politics internationally is the interaction between nations in the diplomatic sphere and the blocs which they act through to act and respond to one another; if any sort of interaction even in a sandbox NRP that doesn't become relevant to everyone in some way is gone then so will the purpose.