another little thing that might have prolonged the empire:
Civilized Germanics: throughout much of the Empire's history, huge amounts of wealth came into Rome in the form of plunder. Carthage, Greece, Egypt, and other rich, civilized opponents fueled the Roman economy. Not to mention, of course, the slaves.
When Rome runs out of civilized enemies, things start to lose steam (you had to spend money to feed the plebes of course, and to maintain a hard-pressed army, and don't forget grand public works). And the Celts fight much harder than they work as slaves.
Now if the Germanics have settled down more and are rich softies, the Empire can keep chugging along, perhaps long enough for the Eastern Empire to save it. Then the Empire can push northward to conquer the Vikings, learning a great deal about seafaring in the process.
This allows for new rounds of expansion down the coast of Africa, against flourishing gold and salt trading empires like Mali and Songhai. Eventually, the Romans get to America, and the Romans of perhaps the 13th century become like Spanish of the 16th, hauling tons of gold and silver from the Americas.