On the Athens faction description page, I read:

Forever influential, but still not fully independent of Macedonian rule, Athens has the opportunity to break free and potentially found a third Athenian League, asserting authority over long term allies and adversaries alike, such as Sparta, Thebes and Corinth.
Is this merely 'flavour text', so to speak, or will Thebes and Corinth be included as factions in the game? Only, there doesn't appear to be room on the campaign map for them...