Let's see, countries to be added to WS:
Definately need Eastern European representation, Russia and Greece have been mentioned many times. I know FumiousBoojum had a post earlier with a little concern that Greece is known more for its mythology than its modern reality. But that can be incorporated as a stepping-off point, as mythologies are used in the Norway and Mexican pavilions. A couple other Eastern European countries to consider: Hungary and Turkey. Turkey had its Ottoman Empire, and culturally/culinarily, is very similar to Greek and Middle-Eastern. Hungary is also a very old region and a crossroads of cultures - from Atilla, and gypsies, and the Gabor sisters (just kidding). I work with many Hungarians - they are all proud of their country - and the pictures they show me of Budapest and the rest of Hungary are beautiful. (just don't call them gypsies - that's like calling an African-American the N-word here in the US).
A South American Country - Brasil in a year-round (and toned-down) Carnivale, like Germany's year-round Oktoberfest, is the most likely representative. It could even have an Amazon trek dark-ride. Some of the other South American countries would be less possible because they might turn out too much like Mexico (like Peru and its Incan roots).
Polynesia - I know it's a conglomeration of many island nations, but it would still be an interesting "country," with a blending of the island cultures. The CMs wouldn't have to be from one single country (Maori, Figians, Hawai'ians, Filipinos, etc) And it could be at least partially funded by the US.
A few countries not to be seen in WS:
Spain and Switzerland - already enough Western European countries. Also, Switzerland would already be too similar to Germany.
Australia - Less chance of it being given a better treatment in AK. Kind of like I think I've read once that the only African presence in WS (besides Morocco) is the Outpost, because Africa is represented by a whole land in AK.
Unless the WS treatment would center around British Empire Australia (Sydney, Brisbane, etc.), and the AK treatment centered around the Aboriginal Australia (outback and New Zealand). But that would be silly to separate the two aspects of a single country, and seen as segregation to many here in the US.
And Antartica - it's not even a country, it's territories divided among many countries. And what would you do with it? Fake snow, a couple penguins, a research station, and nothing else. For that, just bring back IceStation Cool.