To be honest, I don't think the game needs and expansion, but a reapplication.
With what Paradox has done with Rome, I think the game's mechanics coulld be applied it to other periods and geographic locations. A feudal Japan game, using Rome's engine (particularly the civil war model and maybe a bit of CK) as a base? What about a historical game set in India? A particularly turbulent era in China's history, perhaps? How about a stand-alone Dark Ages game covering 500 AD to CK's beginning?
Personally, I don't think that a Roman Empire era expansion has a lot going for it. It seems like it would be the sort of game where you wipe out Parthia and Numidia, and then sit around exterminating barbarians and waiting for civil wars for a few decades. An expansion covering the previous century might be interesting, but I wonder what that would do to the tech tree of the game.
Of course, given some of the cool advances in game mechanics we've seen in Rome when compared to EU3, the engine upon which its based, I wouldn't be surprised if the developers have figured out some new tricks to be applied to some future project we know nothing about.