Greece sounds nice, it also sounds like an awesome place for a ride... although I dunno if it'd be considered overdone seeing as IoA has Poseidon's Fury and Sea World has the Atlantis thing... Hercules comes to mind. Hmmmmmmm...
Russia sounds like it'd be a very good place. Restaurant, Shop, Show. It's got all the potential for a pavillion.
I'd think that India, Africa and such would be good, but they sort of have that covered with Animal Kingdom... they'd sorta be repeating themselves.
My GF said Austrailia, and then we went on about how that'd actually make a cool section for AK, not Epcot... go figure. Like, a desert theme. The Nemo show would fit there too.
Something South American too.... Brazil, Peru, then again it's pretty much the same mix of native/spanish influence so it'd be Mexico all over again, at least conceptually speaking. Although it'd be cool to have somewhere where Emperor Cuzco could be used other than Coronado Springs.
That could switch off with AK too.... a Rainforest section. There's a Rainforest section but not Rainforest area, go figure. They either need to go through with Beastly Kingdom or make that place into an Amazon biodome (also my GF's suggestion. She's so smart. She needs to post more often. Yeah, I'm talking to you. I know you're reading this. Post moar!)