So, in a nutshell:
What if, sometime in the early fourth century A.D, a Western Roman Emperor, noticing the degeneration of the empire and the growing threat of the Germanic tribes to the north, decided to "abandon" distant holdings of the empire, such as Brittania, Hispania, Belgica, Africa, and northern Gaul in order to bring all the legions back to Italia proper (South of the Alps, west of the Danube) and consolidate Roman military might to the defense of Rome itself..
Given this situation, could Roman society survive the barbarian kingdoms, such as that of the Lombards, and avoid being plunged into the Dark ages? Perhaps they would continue existing as a powerful, united state as the Eastern Roman Empire had.
If a consensus can be reached, I might make a time like like the one in my sig.