You could have the White Huns defeat Persia (Sassanids) and then move on to the ERE. They manage to conquer most of the ERE's Asian parts but dissapear like most nomad empire's do. Then the Huns (under Atilla) invade. Pushing the Goths to Constantinople, the Goths are defeated in the siege of Constantinople and accept Foederatii status. They are transported to the others side of the Bosphorus and have the Imperial order to seize back the provinces of Pontus and Cappacocia. Then the Huns cross the Danube river sacking the Balkans and conquering Constantinople after the gates are opened after 2 years of sieging.
With this the ERE is gone, and with a competent Emperor (if youre lucky) in the West the WRE can survive in Spain,Italy,North Africa and some parts of gaul while most Germanic tribes are at war with eachother or trying to get a piece of the ERE pie.