You guys are all amazing! Thanks sooo much for all the replies! Sometimes it is hard to balance teaching and researching and this really jumpstarts my work! Up votes for all!!
Here are two ideas which came to mind.
Rome was an unusually open society for the ancient world. From the very legendary beginnings Romulus opened an asylum and accepted all comers to his new city. Later Rome gave citizenship to deserving foreigners, whether Greek, Gaul, African, Syrian – whoever. To be a Roman was not (entirely) based upon ethnicity.
But Rome still had prejudices against foreigners at times. Accepting Gauls into the Senate for the first time was a huge scandal, for example.
I think their openness was the key to their growth and stability. They integrated conquered elites into their society. This is also relevant today. Modern Western societies are going through this same process of openness and prejudice.
The Romans were extremely practical people. Even one Roman laughed at the impressive but useless pyramids and pointed instead to how their aqueducts were so much better. We also have the impressive road system, public buildings like the Pantheon and Coliseum, canals and sewers.
It's very important, when asking historians questions, to delimit a timeframe. The Roman civilisation lasted independently for two millennia, which is too vast a timescale to really to talk about in a succinct way. Did you have a particular period in mind? The Republic? The early empire?
While their skill makes hiring them an attractive prospect, in too great numbers Germans are dangerous and not to be trusted.
But in all seriousness, probably the best "big concept" to use Rome to talk about is Empire, because the Romans themselves talked about it quite a bit. One of the most famous passages in Roman literature comes from Vergil, Aeneid VI:
Others (I can well believe) will hammer out bronze that breathes
with more delicacy than us, draw out living features
from the marble: plead their causes better, trace with instruments
the movement of the skies, and tell the rising of the constellations:
remember, Roman, it is for you to rule the nations with your power,
(that will be your skill) to crown peace with law,
to spare the conquered, and subdue the proud.’
Am even more explicit praise comes from Aelius Aristades, a Greek intellectual living in the second century, who wrote an oration to the Romans:
As vast and comprehensive as its size is, your empire is much greater for its perfection than for the area its borders encircle.... The entire civilized world prays with one voice that this empire endure forever.... For of all who have ever gained an empire, you alone rule over free men.... You, who conduct public business throughout the whole civilized world exactly as if it were one city-state, appoint governors, as if it were by election, to protect and care for the governed, not to act as slave masters over them.... One could say that the people of today are ruled by governors sent out to them only to the degree that they wish to be ruled....
You have divided into two parts all men throughout your empire... everywhere giving citizenship to all those who are more accomplished, noble, and powerful, even as they retain their native-born identities, [Aristides, for example, retained his citizenship in the Anatolian city of Smyrna while simultaneously possessing Roman citizenship] while the rest you have made subjects and the governed. Neither the sea nor the great expanse of intervening land keeps one from being a citizen, and there is no distinction between Europe and Asia.... No one is a foreigner who deserves to hold an office or is worthy of trust. Rather, there is here a common "world democracy" under the rule of one man, the best ruler and director...
But there were others who took a difference perspective, such as Tacitus in "Agricola" 30, in which he writes a speech from the perspective of a British chieftain:
“Whenever I consider the origin of this war and the necessities of our position, I have a sure confidence that this day, and this union of yours, will be the beginning of freedom to the whole of Britain. To all of us slavery is a thing unknown; there are no lands beyond us, and even the sea is not safe, menaced as we are by a Roman fleet. And thus in war and battle, in which the brave find glory, even the coward will find safety. Former contests, in which, with varying fortune, the Romans were resisted, still left in us a last hope of succour, inasmuch as being the most renowned nation of Britain, dwelling in the very heart of the country, and out of sight of the shores of the conquered, we could keep even our eyes unpolluted by the contagion of slavery. To us who dwell on the uttermost confines of the earth and of freedom, this remote sanctuary of Britain’s glory has up to this time been a defence. Now, however, the furthest limits of Britain are thrown open, and the unknown always passes for the marvellous. But there are no tribes beyond us, nothing indeed but waves and rocks, and the yet more terrible Romans, from whose oppression escape is vainly sought by obedience and submission. Robbers of the world, having by their universal plunder exhausted the land, they rifle the deep. If the enemy be rich, they are rapacious; if he be poor, they lust for dominion; neither the east nor the west has been able to satisfy them. Alone among men they covet with equal eagerness poverty and riches. To robbery, slaughter, plunder, they give the lying name of empire; they make a solitude and call it peace.
I'm a historian of both the first centuries. In that period (leaning earlier) I would stress Rome's treaty system - bilateral treaties with each city-state of Italy that Rome used to (do the same thing we do every night, Pinky) TAKE OVER THE WORLD! The treaties had different conditions and obligations for different states, and awarded different levels of citizenship. It was the first system of its kind in history and is fundamental to Rome's expansion across the Mediterranean. It was the basis for including provincials as citizens and culminated in Antonine Constitution (inclusion of all free people in the empire in the citizenship) of 212 CE.
As someone interested in the history of ideas, the Roman concept of libertas was utterly groundbreaking and underpins our modern account of freedom (as it was drawn upon heavily in the Enlightenment). Libertas designates an equality before the law, binding for plebeians and patricians alike; it was constituted by a collection of personal liberties supported by law, and based on the distinction between slave and non-slave. Its greatest institutional manifestation was the office of tribune, whose job it was to check the power of the senate and magistrates. It is where we get such an emphasis upon individual rights from.
You'll probably want to stress the geopolitics, as the legacy of its empire is still a mojor factor in the workings of the Mediterranean region. Your next unit will also likely be on its successor states, so Egypt and Byzantium at least will deserve mention.
You guys are all amazing! Thanks sooo much for all the replys! Sometimes it is hard to balance teaching and researching and this really jumpstarts my work! Up votes for all!!
Here is an except from a comment by /u/RobsterCrawSoup that is pretty good:
"[...] the history of the Western Roman Empire is full of lessons that remain relevant for modern societies. Above all others, the decay of the Republic into the Pricipate is the original cautionary tale for democratic societies. In addition, and indeed also related to the preceding, there is the cautionary tale of Rome's transitions from traditions of civic virtue to personal ambition and greed, from stoic modesty and honesty to corruption and decadence, and from civic order and participation to bread-and-circus populism. To some extent, Roman writers often gave their Republican past more credit than they should have, but there is more than a shred of truth to the narrative of decay of Roman society. Some of the founding fathers of the US in particular were quite enamored with Roman republican traditions."
Perhaps the most quoted statement about history is that those who do not learn about history are doomed to repeat it. Learning about the evolution of Roman politics, the underlying tension between the patrician and plebeian classes, the impact of political violence, and the decay of the political institutions of Rome are all some of the best examples of civics lessons through the study of ancient history. A lot of this may be lost on a 6th grade crowd, but I don't really know what that grade level can comprehend. However, in the United States especially, where many of the parallels that can be drawn between the political ideals and institutions of today and those of the Roman Republic are not an accident, teaching Roman Republican history is one of the easiest subject areas for getting your audience to think about their own society in new ways in light of knowledge of the past.