Apart from the more obvious choices such as Rome, the Greek States etc I am personally quite interested in playing
Atropatene and
Parnia.
Atropatene was an Median/Iranian state that existed mostly independently of the Seleucid Empire. They ruled what was the upper half of the de jure realm of Media and the Seleucids held onto the lower half. I am curious to see if they conquer all of Media from the Seleucids if they will be able to form
Media itself and then go on to form
Persia should they conquer enough territory.
Parnia is one I am interested in playing too since they are also an Iranian tribe who historically went onto the form the
Parthian Empire which is something I expect they will be able to also do in
Imperator: Rome. The Parthian Empire was the principle rival of the Romans up until the 3rd Century AD and their unique use of horse archers made them a force to be feared.
I have chosen these mainly Iranian nations as recently I have taken an interest in the history of Ancient Persia and really look forward to seeing what options there will be for that region in the game, principally what formable nations there will be, how their religion (Zoroastrianism) will work and any other unique features empires of that culture will have available to them.
I know a possibly obvious option would be the Seleucid Empire which I do plan on playing at some point, but I would rather play a nation that is native to that region such as the ones I mentioned, as the Seleucids are Hellenistic and I would rather play Macedon or Sparta for that particular experience.