It will be a great day indeed when Toronto allows "immigration free" transiting.

It will open up a market of 280 million potential people (USA) to Air Canada's international network to Europe and Asia. The current situation of arriving at the USA terminal, going through the smoke and booze cops (customs), catching the shuttle bus to international with luggage is no way to run an airport. It certainly turns me off when I do the Atlanta - YYZ - LHR run. (Which is why I am doing the Delta thing right now).

I would predict an increase in traffic directly as a result of this. Toronto has the potential to be a huge North American hub. Air Canada's RJ and A319 network into the USA will certainly pay off when the new terminal opens (assuming that USA and International will be in one terminal.)