Just for background, if i remember right, the classical Greek age ends by the fourth century, when Alexander the Great starts his conquests. This begins the Hellenistic age, which is culturally distinct from Greek. From memory, i can't recall when the age stated.
As for ideas, again go with the classics: A curse is on their city because someone within it (perhaps a PC) as sinned (though you may not want to bring in incest....)
another: a race of creatures live in a cave and think that is their only reality, until one escapes to discover the real world. He is probably perceived as a threat to the surface people, and when he returns he is not believed by his own. the PCs must act as a) intermediaries to keep the visitor from being killed (or perhaps they lead the lynch mob) or b) evidence that the visitor isn't lying/mad when he goes home. Obviously, I'm taking some extreme liberties with Plato's Allegory of the Cave. Plato has a lot of good ideas. There's also the story of the man who finds a magic ring while plowing his field. the ring can turn him invisible, which he then uses to rape the queen, kill the king, and rule the city. somehow the PCs discover the plot and must stop him/convince him to live virtuously.
note: I erased a lot of this because i realized I was just being contentious, and may have even had a number of innaccuracies in my phrasing, it wasn't important anyway.