One thought I had on Russia is with their decoration of non Russian's to Drakia before the Regression vast swaths of Eurasia were left depopulated. Leaving it wide open for resurgent horse nomadic cultures, such as the Cossacks from the west and Mongols from the East. So I can see them being a nightmare for the surviving Rhomanians/Greeks in the Black Sea.Alternatively, them Rhomanians could have been one of the frontlines and targets from that supposedly apocalyptic war, consequently making much of the Balkans... accursed. Such a status would disqualify the region from being the center of power for the resulting rump state.
Alternatively, they could make for such a society in a comparatively underdeveloped and under-targeted Anatolia (if they ever manage to centralize), although its rurality and bonded population would have more in common with a Spartan society than anything Roman. which with history fading into legends they would all but compare themselves on quite humbly.
Rump "Sparta" would still have what's left of Russia behind it's back however, which would make for a very good leverage in the Black Sea and even beyond. If they're smart enough, they would consider the Black Sea as its base of power that it should develop and integrate, not some comparatively devastated Mediterranean territories that would like to have nothing to do with their practice of conscription and bondage. They may even look down on the "Barbaric Europeans" that make up whatever's left of Western Mediterranean (including Spain), with the "Democratic West" (maybe those Western Europeans could be quite similar to the Vikings, perhaps with the raiding reduced) continually contesting "Sparta's" rule by, well, raiding.
EDIT: If anything, it would make one hell of a JRPG fantasy setting, and even an alternate and more fleshed-out setting for Attack on Titan. Easily-excused rivalries, a continent-sized political struggle, admixture of different techniques originating from the different culture, pray tell, are all these really incidental?
Consider the world was thrown back to the dark ages Europe has probably scene a return of Feudalism. Alongside new Medieval castles being built upon the remains of twentieth century fortifications left over the Great War and Silent War.