Russia would be great. In addition to unique cultural facets, a replica of St. Basel's Cathedral in Moscow would make a great architectural addition to the look of EPCOT.
Greece is another good choice. The acropolis/parthenon and other various temples/architecture would look great. The mythology of greek gods (tie in to Hercules), etc. would make this another good choice.
Australia would be a wonderful addition to
Animal Kingdom. There is lots of indiginous wildlife unique to Australia that could be featured. Everyone says AK needs more rides and they could build a replica of "Crush's Coaster" from
Disneyland Paris that is tame enough for kids but wild enough for coaster lovers and this would tie in to the great barrier reef, the EAC, and of course another Disney character (Crush/Nemo).