Interestingly, my wife and I had this very conversation a week ago.
We agree that Europe is over represented, but the issue is figuring a country or countries that:
1. Offer culture substantially different from the already represented countries to warrant the construction costs.
2. Offer shopping and ride options to attract enough patrons.
3. We're unlikely to go to war with in the forseeable future, or have not been at war with in a long enough time.
4. Offer culture that is not so substantially different as to be offputting to a predominantly American and Western European audience.
So most Arabic countries fall out of this category for reasons 1 and 3. Central African countries might qualify, but one could argue that AK represents them quite well, as well as AKL. All 3 large North American countried (US, Mexico, Canada) are represented, but no Carribean or Central American countries are. And no South American countries.
So here are some ideas.
Polynesia - While not a "country" it represents a collection of cultures that offer something different from the regular stuff. Any one of the major islands or island groups can be represented, and fit well in the troipical atmosplere of central Florida. While represented at the POLY resort, I think few will visit the resort compared to the many who'd see it at Epcot.
India - Very different from the offerings at Epcot, Indian food can be a bit much for those unaccustomed to it, but joy for those who love it. India has a rich and complex culture, and plenty of cheap goods from the "other" country with over a billion people.
Antarctica - While not a "country" in the traditional sense, Antarctica allows Epcot to expand on the Ice Station Cool concept. I picture it in the old Millenium building, appropriately cold and populated with a small zoo of polar bears, penguins and walurs. In addition, Disney could use this as a teaching area about global warming, environmental science and ozone depletion. They could build a South Pole research station not unlike Sea Base Alpha so the whole building wouldn't have to be kept at freezing. This is probably my most ambitious idea, and could house an E-ticket ride or somehting like that.