mmm, australia would be fun, with all sort of deadly creature, giant spiders... oh wait, isnt that modern pre-war australia ?
ok, Canada would be fun too. first, they could implement a season system, so that half of the year you have cold and snow that change the landscape. so now, you need armor, but also winter gear to survive the cold, but not all the time, just in winter. plus if you make water freeze when it's cold, it could radically change the way you travel or the way you are doing quest depending if you do it in winter or in summer. and mutated polar bear beats deathclaws...
it also would be interresting how the politics play in canada too. yes canada was annexed but only in january 2076 according to the fallout timeline. so right after the bombs, probably that a part of the survivors would have turn toward what's left of the USA, but another part would have try to rebuild canada. there's a chance of civil war just after the bombs there...
Florida should be something to look out for too.