Sorry to take too long to answer. I'm on limited time. Any way, thanks for all your thoughts and answers.
I've always had this idea that with the borders secure, the barbarians quite and the empire stable, an emperor, in an effort to keep a general from challenging him, would send him to Egypt to find the source of the Nile or be executed. So the general would raise up an expeditionary force, negotiate with the Nubians and other powers to reach the swamps of Southern Sudan. There he would skirt the edges of the swamp and would then follow the Nile until he reaches Lake Victoria. Upon reaching the lake and concluding that he has reached the source of the Nile, the general would then retrace his steps and go back to Rome and depose the emperor. Once an emperor, he expands the empire from that foothold. Of course this doesn't take into account of the diseases his expeditionary force might face.