I think seeing what happened in Canada, even in some sort of DLC would be interesting, (me being Canadian =3). Of course, going Mad Max in Australia would be pretty sweet.
I do have to say, that it's about time we see whats going on in China, you know, the guys that nuked the States. We know next to nothing about them.
If were staying in the states, Nevada would still be cool. Area 51, New Reno, lots of areas to explore. Otherwise, New York might be interesting, if touchy. Since the twin towers would be seen destroyed in the game perhaps, and walking a blown out times square might be too much for some people.
The talk of how advanced the tech in the Commonwealth is intrigues me, that would be a cool DLC, and seeing California again would be interesting. A 3D Hub and LA and Hollywood would be cool. (I havent played the old ones much, I plan to beat them and tactics shortly.)