If I were to add new major civs, and for that I'd have to figure out a way to re-use civ-slots that aren't needed in the 600 AD scenario, I'd go more for which parts of the map are still empty, and usually not that much subjected to colonization - that's why I could imagine a major civ in Indonesia (even though the Khmer and the Japanese move in there sometimes, and some very few times even the English and the Spanish).
Israel as a major civ wouldn't really be under consideration, as it would require a free slot in the 3000 BC scenario, and would be situated in an area more crowded than Europe.
Other civs I would consider though are Italy, a 600 AD India, a 600 AD Persia, another north-african civ probably representing the Berbers, a 600 AD Ethiopia (or another central or east-African civ).
However, all this will take a long time to implement, as I'd have to find a mechanism first that allows me to use the slots for another civ, as I'd have to rewrite (or rather, re-direct) most of the code to do that.