IMHO, a more plausible scenario would be for the Germanic/Steppe migrating into the Balkan provinces (likely the Huns, with the Ostrogoths, Gepids and others orbiting their dominion), they can successfully overrun Illyricum and Moesia, perhaps going as far as Greece like the Slavs and Bulgars did IOTL. Constantinople itself might be a nut harder to crack, and I believe that a rump Roman regime might succeed in preserving Thrace and some European places such as Thessalonica (much like OTL 1400s Byzantium).
Now, the greatest threat to the ERE was and will ever be Sassanid Persia. The ERE could afford to lose the Balkans, but its collapse IOTL was almost precipitated by the invasions from Asia, at first from the Persians, and later from the Arabs. ITTL, Persia will take advantage of the anarchic state of the empire and can waltz into Syria, Palestine and Egypt after some campaigns. After all, the eastern subjects despised the Imperial regime of Constantinople out of theological, cultural and social disparities. I think the Persians are (at least on probabilistic terms) more likely to take and hold Constantinople and whatever Roman remnant there is than the Germanic invaders if they succeed in annexing Anatolia. This also pressuposses that Persia will have no interest in going deep into Europe, avoiding the path of the Ottoman Empire, for example, but they might become a soft hegemon of sorts over this new "barbarian" order in southern Europe.
This earlier collapse of Rome might spell a new golden age for Persia, especially if we butterfly Islam. Also, I expect, in any case, that the post-Roman polities in the territory of the former ERE might tend to develop quicker than those in the WRE.
Depending on the butterflies, of course, we can have the "barbarians" successfully conquering the Balkans and then pouring into Asia by the straits. I sketched some time ago a scenario similar to this, in which the Goths migrate to southern Europe instead of going to the WRE, followed by the Vandals and the Huns. Sooner or later, they will meet the Persians coming in the opposite way, and only a more detailed analysis of the scenario can say if they might triumph and establish their own kingdoms in Asia (likely balkanizing Anatolia and perhaps a part of Syria and Armenia).