(OOC: Consul/regnal years were the preferred system when Rome was still a viable state. AUC was a fairly secondary method of dating, but could easily move to the fore once Rome was gone. Of course, one could just as easily see years dated from when Rome fell as opposed to it's founding, but that would probably a require a more definitive date for when the Empire fell than OTL.)
I would imagine that a hypothetical Roman Empire might not be so inclined to foreign adventurism as the Republic was; one of the best ways for any up-and-coming politician in the Republic to make a reputation was through battlefield victories and conquest. In a monarchy, the heir would have no need to do such a thing; he would be assured of his position from birth.
Also, I would say that the acquisition of Germania is undervalued by a lot of historians. It did shorten the Empire's borders, and the Germans themselves became the backbone of the Republic's legions.