If we're to continue the Roman analogy, an Augustus is a long way off. Roman imperium started under the Republic, we should remember, just as the Athenian empire expanded massively under the democratic government. In the case of Athens, they overreached, and paid for it dearly, as their enemies, Sparta and Persia, united, razed the city and installed an oligarchical tyranny (the reign of Thirty Tyrants). In Rome, increased militarization of their foreign policy and the close links between riches, military command and political power, along with an increasingly disgruntled lower class (whom certain Roman factions were entirely against gruntling) led to a civil war. As you point out, Augustus was smart. He realized that continued expansion would lead to continued social unrest and produce ambitious challenges to his rule.
Of course, the current system is not exactly the same. But there are parallels. The military-political link is much weaker, but the commerical military and political commercial links are so very much stronger. An economic crisis is threatening to sink the middle class, making paupers out of much of the population.
The question of limits may not be up to America for much longer. China cleaves to its territorial sovereignty, even as the economic forces it has unleashed in the country seek to undermine it. A backlash will not be far in the coming, and the angry, mostly workless young men of eastern China, filled with hate for America and Japan, may soon be swept to power on a tide of populist rage and xenophobic nationalism. Russia too, has demonstrated that American control of the international system effectively stops at its borders. Furthermore, it pounded on an American military ally for a week longer than necessary, just to show it could. Missiles are being placed in Kaliningrad in a couple of weeks, which the Russians claim effectively counters the American missile defence system. Whether this is true or not is immaterial. It shows a dangerous militarization of Europe that has not been seen since the end of the Cold War.
Augustus had the benefit of choosing his limits. He emerged from a ruinous civil war, from which few powers could take advanage. America will likely not face the same sort of crisis, but equally not be not so lucky.
