I don't think Tibet would produce enough of a scandal for people to care. The limit of players in China that play Civ 5 is telling - and you would simply focus the civ on the 7th century Tibetan Empire - not the pseudo modern Tibet people hear about.
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There have been more controversial stuff in game (IE Stalin) - I honestly don't think Firaxis cares about it too much.
That being said I would like to see them in.
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A couple of choices for me
Israel
Tibet
Comanche/Apache/Anasazi
Zimbabwe
Morocco
Portugal
Indonesia (Rather than the Java Kingdoms or Majapahit)
Kongo
Brazil
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I already have a name for this Expansion worthy list of DLC (Obviously its ripped off of Jared Diamond - but hey why not have Jared Diamond voice overs)
Civilization 5 :: Guns, Germs, and Steel.