I have an idea for you, building off of what someone mentioned earlier, with the children of gods and goddesses being pretty popular.
What if the whole Pantheon suddenly disappeared? Not even so much as a cosmic "I went out, be back later" sticky note.
Maybe the Titans have come back, and are exacting revenge. Perhaps a lack of faith has seen them slowly wither and die. Maybe that nefarious Trickster, Loki, made them vanish to soften up the area for a hostile takeover. Maybe Prometheus got free, and had to show them all just what fire is capable of doing.
There's probably a dozen other alternatives, and it need not be readily apparent. But regardless, there was one thing the Greek gods and goddesses seemed to be known for: Getting it on with humans, and siring heroic demi-gods.
At the same time the main Gods and Goddesses disappear, these minor people, who can be just about anyone from anywhere has a sudden epiphany of understanding, of who and what they are, of what they are capable of, and just how badly things are screwed up. It then falls upon them to fix, or take advantage of, the situation.
Maybe the new minor Gods and Goddesses will find some way to absorb the essence of another deity, and transcend themselves, assuming the role that deity played. Perhaps they can free their parents, from wherever it is they have been kept. Maybe they'll just take advantage of the fact that they are quite possibly the strongest entities in the world right now. You could take it in many, many different directions; and who's to say that each of the demi-god's won't have their own agenda?