So I guess the basic question then is what defines the Roman Empire, and who is Roman?
Anyone who was integrated into the empire and has not broken away from it is Roman IMO. For instance, Byzantium wasn't the Roman homeland, but it was conquered, integrated, and ultimately never made the decision to leave, though the decision was eventually forced upon them.
Similarly, if, say, through ASBs, Russia were conquered by the Byzantines in 1350, was reasonably integrated into the empire, and then Constantinople and Greece fall on schedule, leaving the imperial court to migrate to its territories in Russia, territories which it has already controled for some time and are culturally more or less similar to the ones the court is leaving, then we could call Russia a direct continuation of the empire. It is no exaggeration to call that scenario impossible, which is why Moscow can't be a third Rome with a 1350 PoD, but that is what it would take.