The only way that could ever happen would be if Detroit seceded from the United States and became part of Canada.

Not too many Canadians are big on the idea of annexing new territory. Many of them feel it represents hegemony, as outlined in the writings of Pierre Trudeau. Trudeau was a genius, but I think he was wrong in this case. It's only hegemony if the annexation is forced upon the weaker party by the more powerful party.