This is very good news. I've been hoping for an Assassin's Creed: Greece for a very long time. It's the perfect setting to really push the First Civilization plot points and move the story forward tremendously rather than stringing it along.
As for the main character I am hoping for a female protagonist being the daughter of Atlas. Elektra or if they really want to go tragic, Atlas's wife Pleione AKA the hidden one. Zeus plucked them into the sky. All of the seven sisters are named after birds and fits the themes of Assassin's Creed as well.
Thematically Greece is the perfect fit for Assassin's Creed far more than most of the recent entries have been. As for why it's taken so long for us to get a game set in Greece, perhaps it was planned to be used as at a later date with the First Civ and the Greek Gods. Something that is now being pushed a little harder. I'm curious to see where this goes. I have much higher expectations for this entry should this rumor be true.