Now you wouldn't have to make all of Canada, just some of the major cities, a lot of wilderness between them and it would work beautifully. Now if I've read my Fallout lore correctly most of Canada survived the initial nuclear war because it was too important to both sides. There is a lot of potential for a great exploration game with some genuinely unique Canadian humour thrown in. The game would have a much more inclusive and comprehensive survival mode that Fallout: New Vegas introduced.
Some features I think that would make Fallout Canada unique from other fallout games while still holding true to the series:
-Weather effects (I'm talking out and out seasonal changes here that change the way you play the game. You are in the great prairies. In summer time prepare for some blistering heat, flash rains, and even high winds knocking your visibility down to nothing as you cross the vast plain. Oh and mutant buffalo. In Winter this changes completely. Where once you were able to travel pretty much willy nilly now you have to contend with gigantic snow drifts, the cold, and blizzards. Oh and mutant polar bears. Yeah temperature changes would be added to a survival mode. If you don't dress for the weather then you are going to find your temp meter depleting rather quickly...)
-Vehicles (They can use this as a platform to finally introduce vehicles and mounts [moose/horses] into the game...if only so you can experience your vehicle stalling in a Canadian winter.)
-Different languages (The French/English part of Canada is a pretty big part of the culture. It would be interesting to have different languages that your character could learn so that he/she could get along with different factions. This could also be a way for Bethesda to gauge reactions for Fallout: Asia)
I think it would be a great idea and a great addition to the Fallout franchise. Fallout has primarily explored different aspects of America but that's quickly getting old and stale. Why not try branching out into a different country in your future game? Canada is a great way to do this without alienating crowds by going TOO far away.
Thanks for your time.