You can try it differently - instead of speeding the fall of Eastern Roman Empire, make that Western Roman Empire survives. It is one of the ironies of history, y'know - after V C. barbarian invasion in OTL there weren't any external dangers of that scale to the region that WRE occupied afterwards... While ERE/Byzantium had to deal with Arabs, Bulgars, Slavs, Turcs, Piechengs...