Something that we should consider is that Canada was annexed into the U.S. in 2054, according to Fallout 3's load screen. Bethesda could still make it in Toronto or Ontario I suppose, or they could have the war of 2054. That would be fun.
Toronto is in Ontario, so it would have to take place in and around both most likely.
I would prefer if it took place a bit more North, even if it were in Ontario.
I want a frozen wasteland with all it's icy dangers.