Yeah, we have used a good amount of the big civs, but I think you could still find enough civs to make up a new pack. There's still a good number of either empty areas, or potentially new civs out there. Yeah, maybe not "Babylon"-levels of interest, but there's a lot of NFP civs which I assume are not huge draws. Like, I doubt there's a million people who are like, "Whoa, the Gaul? Take my money!"
I would imagine that something based around Portugal, Sioux/Iroquois, Berbers, Italy/Venice, plus some others like maybe a Carribean tribe, another African nation, etc... I don't think that's necessarily that much less of a draw than NFP is.