A possible trial run could be between the U.S. and Canada. An even smaller trial could be not to allow a hub but to allow ticketing. For example, Air Canada could be allowed to sell Seattle-New York La Guardia seats via Toronto.

For a U.S. airline to make it work, some sort of Canadian preclearance might help. For example, an end of a concourse could be blocked off and treated like Canadian soil. Southwest could do it at Chicago Midway with flights from Toronto, Montreal, Ottawa, and Halifax continuing on to Vancouver, Edmonton, Calgary, and Victoria. For Chicago passengers or other U.S. originating passengers wanting to go to Canada, they could clear Canadian customs in Chicago. An alternative would be to force Canadians to clear U.S. customs if connecting but that would slow down connections.

American, Delta, and United would probably not want to offend their partners, Westjet and Air Canada. That would leave Southwest as the most likely to be interested.

Recently, I tried flying between a U.S. city, Toronto, Vancouver, and driving to the U.S. What a long trip! It took about 15 hours each way versus 6-8 hours if not through Canada.