Just as an idea for a Roman Empire that dominated the known world:
As far as I can tell, it came down to a few major points in history. Much as we call Augustus the mightiest and first Emperor, he also was the emperor that Rome began its decline in.
Now, there were three or four key flaws in Augustus' reign:
1. Augustus attempted throughout his reign to promote ideals that were considered old-fashioned, but he failed miserably.
2. Marriage to his wife Livia was, as far as I can tell, one of his worst mistakes. She married him perhaps with a bit of love, but she most likely married him to further her son's future and to advance her own.
3. Augustus selected Quintilius Varus to defeat the German tribes-- this was his worst failure. If he had selected a better general, his conquests might have been pushed farther into barbarian Europe.
4. This could tie into 1 and 2-- Augustus was unable to find a political heir suitable to the task of defending Rome and maybe pushing it further. Rome might have survived longer if Tiberius was not in charge.
Nonetheless, Augustus still did a fine job of ruling Rome, but I believe these factors tied into Rome's fall.