I never managed to watch all of the first one and I have tried 2 or 3 times. I find there is only so long that oiled up man muscle can hold my attention.
If this film did well at the box office, then they have a ready-made trilogy. Themistocles was Athenian, so the "rising empire" is Athens. The third film could then cover the Peloponnesian War, with an emphasis on Sparta vs. Athens.