It's worth remembering that while the Western Roman Empire fell around 500 ad (if one wants to be generous), the Eastern Roman Empire lasted another
thousand years.
I'm sure if the Western Roman Empire had been healthier, or if in fact the Empire had never split, then Western Europe would have developed very differently. On the other hand empires of that size generally fall quickly, or else become empires in name only. It's quite possible that Rome would have become a figurehead for a superficially united Europe, while the provinces themselves followed a similar course.
Would have been interesting - the Arab/Turk Muslim conquests of Asia Minor, the Mediterranean portion of the Middle East and North Africa would have never happened.
The Roman Empire did lose territory at times, and it's worth remembering that much of the territory that was conquered during that period was taken from the (Eastern) Roman Empire. Despite the parenthetical it was still a power to be reckoned with, although certain matters concerning religious schisms and the fourth crusade probably wouldn't have occured if the Empire remained united.
Unfortunately those topics can't be discussed in depth without bringing up religion.
