Perhaps for a collaboration (or, hec, have it GM controlled) where either one nation gains a little too much power for it's own good and trys to throw its weight at everyone (what i speak of would actually be OP, so make sure the one doing it is mature enough not to complain if other nations gang up to fight, and make sure said person is not afraid of losing, but also who won't just wipe everyone out), or have a group of nations that allign and attempt to form a new order which attempts to squeeze out anyone who doesn't want to join it.

In the meantime, have events such as natural disasters, whatever come up to spice up the gameplay... I don't have a single thing to do with fallout, so not too much I can really offer here.