Because they're so moddable, a total conversion would be possible. I know the mechanics are similar but the game of thrones mod is breathtaking (I recommend this too)!
A good place to start would be the time when Rome was merely a small kingdom and take it from there. Lots of Mediterranean based conflict with other civilisations like the Greeks, Egyptians, Estruscans, Carthage, etc. The end result might not even be a Roman Empire but, however unlikely, a Gaulish empire or a Carthaginian empire. That sounds like a lot of fun.