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.
I guess I can't argue if you've sat down and tried to model it - the culling of establishment structures makes sense now.
Given me inspiration to take a stab, if only to come to your conclusion. A decent, historical early Rome grand strategy game is too much to let go of.
Hopefully an impetus to put some code down on a game I want to play.
Which is not to say I wouldn't love to see it happen, but it's a hard problem.