So the years of 235 to 284 were really not kind to the Roman Empire. Nearly bankrupt, facing internal strife, external invasion, plague, and near total collapse of the bureaucracy Rome honestly shouldn't have recovered. At one point it was even split into three different empires, the Gallic Empire that control Spain, Gaul, and Britain, Rome which had Italy, Greece, and the Balkans, and the Palmyrene Empire that controlled Egypt, Syria, and parts of Turkey. There were something like 25 Emperors during this time and most ruled for less than a decade. So what if Rome doesn't get back on its feet and the system totally collapses with the three Empires becoming successor kingdoms to Rome. What would be the biggest changes to the Western World? The biggest I can think of is that Christianity might never rise beyond a popular lower class cult as there'd be no legitimizing force like Constantine to spread it all over the Empire.