Does this sound realistic:
Aurelian is killed in 242 in battle against the Sarmatians. No major changes for a few years until:
256 AD: Major Gothic invasion in Asia Minor. Dacia is lost as well. In the west, the Franks cross the Rhine and the Alamanni reach Milan. Goths appear in Greece. In Africa, Berbers massacre Roman colonists. Persian Empire conquers Mesopotamia and Syria. The frontiers of the Empire are crumbling.
257 AD: Rhine defenses are overrun, barbarians flood into the Empire. The Goths separate into Visigoths and Ostrogoths. Roman forces are defeated along the lower Danube by Goths. Persecution of the Christians intensifies.
258 AD: The Goths ravage Asia Minor. Gaul, Britain and Spain break off and form the Gallic Empire.
259 AD: Valerian is captured and executed in Persia. Postumus becomes Emperor of Gaul and drives the barbarians from the Gallic Empire. Italy is invaded by Alamanni.
260 AD: Gallienus becomes emperor. He successfully defends Italy against the Alamanni. Syria, Egypt and Palestine break off the form the Palmyrene Empire.
261 AD: The Alamanni are defeated at Milan. Gallienus restores Dalmatia to the empire.
262 AD: Without the rich provinces of the east, the Roman economy collapses and inflation skyrockets. The Gallic Empire is spared the worst of the crisis as it has a more valuable currency. Postumus defeats the Franks in Belgica. The Temple of Artemis in Ephesus is burned down by the Goths.
263 AD: Postumus' success in Belgica is short lived: the province is overrun by the Franks. Gallienus defends Italy from the Alamanni once more, and once more he is successful.
264 AD: Odaenathus, king of Palmyra, conquers Edessa, Nisibis and Carhae. Seeing the weakness of the Rome, he breaks all relations with the Empire.
265 AD: The Ostrgoths invade the Balkans. Gallienus defends Dalmatia and Illyricum, but Thrace, Macedonia and Dacia all fall to the barbarians.
Thats what I have so far. With Aurelian dead, Rome wil have lost one of it's greatest generals of the time. Seeing as I did this in an hour, it is probably full of errors and its not all that different from OTL so far. The main change will be that Aurelian will not reunite the empire.