Speaking of extra leaders, I was always a little surprised that the US wasn't spiritual, given the frequency with which we tend to change our civics.
Anyway, highest priorities should be Babylon, Vikings, and Ottomans. After that Byzantines, Carthage, and maybe another european or african civ.
One thing that I think might be incredibly controversial... is which civs are effectively represented by "Barbarians"? Barbarian is a handy concept for representing a decentralized confederacy of very small nations, but just the stigma of never having the chance of "winning the game" is incredibly dangerous.
In any case, regarding Israel, I'm a little warry of adding any civ that has effectively been a OCC/wandering settler all it's history without winning very many major wars (given that Israel did not keep any territory, I don't think the seven day war counts). Plus, it just seems wrong to found israel and not go with judaism. It's just not worth the controversy.
EU is not a nation; it's a very large open boarders/MPP.
While portugal had many colonies, I'm not sure it's distinctive enough from spain (in history and leader traits) to warrent being in civ, especially when it has been in before.
Korea is a must, given the gamer demographics.
Plus, they stuck it to Japan for a long while.
I dunno about sumeria. It's arguably the most important, being (arguably) the first, but I think you can fold them in with Babylon and nobody would know the difference.
I think north/south america can handle only one more civ, and it should likely represent a native american nation. The iroquois nation was a messy compromise last time, imo, but I think just labeling indian cities barbarian would be unaccetable. I suppose native american representation could dovetail with a bit of Canadian representation, given their particular shared history of peace (relative to the US, anyway).
I forgot that the aztecs were already in the game and voted mexico.
Did the Celts ever actually conquer anybody? Don't mean to be insensitive, just ignorant.