presuming a such far-fetched scenario, the following scenarios seem to be more plausable:
-a couple of centralised isolationist xenophobic and ultra authoritarian and ideologized empires (Rome, Persia India and China, maybe central america and inca) that are not intested, in external affairs.
-tehnological backwardness, as well as great differences betwen the tehs of nations
-severe social unreast caused by overgrown population (wich would have anyway broken the antique nations)
-more smaller more lively "barbaric nations, that, beause of they're competition, are tehnologically advanced, comercial, civically united (germanic nations, slavic, mongolic, tucik, native-american, etc). They dar are bound to succede against crumbling empires, as they did 1500 yeas before IRL.