Turks are a definite.
Babylon next, although the Mideast currently seems to function alright (unless you wanted to play as Mesopotamia).
I'd pick the Celts before the Vikings but I know there is always the question... where do they go? If Rome never conquered Gaul would it be more applicable to have the Celts or French in that region??? They are a few solutions I can think of, none of which are perfect.
I see the same question for a native civ in North America... albiet the vestiges of civilization in the region were slim, I'd rather see America (USA version) be a rebirth of a conquered civ, throwing off the shackles of English rule.
I like the idea of Carthage, the Barbarians in the region are isolated from the others so it would be nice to play them as a seperate civ.
The Viking presence is missed, no raiding parties from Scandinavia, yet what role would the civ assume if intended to live on until the Modern Age.
Korea would be tough to put in. They would need a lot of influences to keep them bottled up in their pennisula. I'd rather view the Mongols or Japanese as representing the Koreans. Of course, I'd rather view the Mongols as "China, North".
Dutch and Portuguese. I would love to see the varied colors of territories grabbed by the Europeans other than Red, Blue, and Gold. On the other hand, I'll side with the fact that "Spain" can easily act as both Iberian countries, etc. Of course, I would like to see more German/Deutsch/Dutch colonization, Indian Ocean stuff.