A Case of "Eastern Roman" Collapse instead of Western one?
Constantine decides to keep Rome at Italy instead of building Constantinople, He try to reform Italian urban populations instead of rebuilding the new one.
A different Barbarian migration made the Franks and Goths to settle in the Balkans instead of Western Europe.
Huns invade levant, ransacking Sassanids cities in Mesopotamia, and push into Egypt instead of Western Europe. Holy Land fall to the Huns, but Roman legions under Fluvius Aetius try their best to bottle them at Gaza and then Suez, which they failed. Egypt is overrun and sacked, Romans withdraw from Egypt and Choose to reinforce Libyan and Carthaginian coasts instead.
After century as nominal foederati under Roman Empire, the Franks and Goths in the Balkans openly rebelled and took with them the entireity of continental Balkans. Roman presence is limited to Greek cities at the south, Athens, Thebes, and Thessaly are made into fortified Roman strongholds to block the tide of Barbarians, while Roman forts at the Rhine river banks, Helvetia, and Venetia is heavily reinforced.
Italian rejuvenation programes start to get on the right footing, helped by ongoing war that ironically threaten incursions into North Italy and Gallia.
Sassanids oust the Huns from Mesopotamia.
Huns settle in Egypt after finding that marching through libyan Desert is nigh impossible, and they can't compete against Roman navy for now.
A Roman Emperor deny primacy of the Bishop of Rome.
Hephtalites invade northern border of the Sassanids.
Slavs migrate to Germania and try to push into Gallia, but held at Rhine borders by Roman forts.
Angles and Saxons, pushed by the slavs, migrate to British Islands, but the better condition at Gauls means the Romans could afford to reinforce the Britons. Angle-Saxon tribes then settle on formerly Pictish lands in the North of Britannia.
Scotti tribes stayed in Hibernia, adopting a distinct version of Christianity.
... Umm gimme a break, but I must re-freshen my mind about pre-Islamic arabia to get some ideas on how they will react to Hun-Sassanid-Hephtalite wars instead of Roman-Sassanian one