How about a law that requires an expensive permit to transport alcohol, tobacco or guns by plane. Then do random spot checks at airports. That way, incoming airline traffic from the USA could be treated as domestic. The limited tax savings on other products hardly make smuggling by plane worth it. So the loss in tax revenue would be very limited.
We could also do the same for traffic from the European Union, Japan and Australia.
Obviously, it would be nice if those countries returned the favour to Canadians (Australia requires an on-line visa, for crying out loud), but the increase in efficiency would be worth it regardless. Planes from the UK or Spain could stop over in Halifax without forcing passengers to de-plane. Planes from the USA could stop in Toronto or Montreal before flying to their final destination (instead of forcing passengers to de-plane in already busy airports)
More importantly, immigration official's time could be better spent questioning people who fly in direct from India, the UAU, etc, people who haven't already been vetted by American and European immigration.
Or, we could continue to live like this.