Even if the audience may not know a lot about the empire, they probably will know about the 'hollywood-version'. No need to tell them anecdotes about the depraved emperors, gladiators etc. They already know this, and you'll only be perpetuating the stereotypes. Use your time to tell them something they don't know. For instance, focus on a period they won't be familiar with, such as the third century, or the fifth.
Avoid the obvious (What was the Roman legacy, why did Rome fall/rise), you won't have the time to deal with such broad topics anyway. Instead, leave them a single, well-made point that changes their perception of Rome, if only just a little. That's what will make them want to know more.