I'm starting to think Greece won't be in as one nation, probably just a bunch of city states, like Sparta and Athens, with the Greek civ being Macedon, since Alexander is Macedonian, and he's really the first pick for a Greek leader. The only problem with this layout is that when Greece was actually around, they did try to conquer stuff, while city-states in game don't. But I am 99.99999% sure that the Greeks will be in in some shape or form.
I am also personally wanting Spain to be in as the 18th, and also think they would be the most likely. Ragnar would be a lot like Askia, and so would Leif Erricson, or almost any Viking leader. Persia has probably been replaced by Ottomans, and Babylon isn't important enough for a top-18 release (Not that I don't like Babylon, they're #4 or somewhere like that on my list of favourite civilizations, historically. I just think that other civs have contributed to history more, since Babylon's history hasn't been documented as much.).