I'd like to see Fallout: Toronto.
You could introduce a new kind of hardcore mechanic regarding the snowy winters of Canada, about not freezing your ass off outside too much. It'd be a nice balance of city and open expanses, too - capturing the awesome balance Fallout3 nailed.
You'd hear a lot about the history of Canada. The annexation, the war at Anchorage, food riots etc. It'd be really interesting to see how Toronto survived and adapted to the nuclear war.
Overall I think it'd be way more interesting then Boston, but frankly as long as it's on the Skyrim engine, I'm happy.
So far, the Fallout games have told stories of California, Nevada, Arizona, Washington DC, Point Lookout and Pittsburgh. Europe is barely mentioned; only with Tenpenny and Moriarty having come from the continent somehow. Mexico is further talked about; Raul speaks of how Mexico City and the surrounding areas were turned into toxic horror lands of gang warfare and pestilence.
I'd like to see a Fallout game take place in Canada because it would be a whole new story, a whole new experience and interesting to learn so much more about the story and timeline leading up to and following the Great War's nuclear exchange. Ghouls talking about the riots and war crimes, survivors misreading Canadian traditions and culture, the CN tower being the power in the Frozen Wasteland, and, Atom willing, mutated moose and beavers.
A lot of people criticize Fallout 3's story of the Capital Wasteland for being separate from the West Coast escapades of the first two Fallout games, but I actually appreciated the whole new experience. Such an experience can easily be replicated in (To)Ronto, Ottawa, Vancouver or even Alaska itself, which could lead to information about China's fate, or even a lead in to visiting the land itself.
So far, all the history we know is that of the United States, even then it's only the above six places. I want to see the rest of the world.