That would make the most sense, but realistically it would have to be a total, crushing defeat for Canada and any British forces sent to save it. Otherwise, due to its long history with the British, I couldn't imagine it not being tied to them somehow.
Maybe a Dominon government more heavily influenced by the Americans, with the post-war Canadian government trying to appease the United States despite the unpopularity it would cause with the Canadian people.
If it's going to go back to the British in some form, then maybe the US will want to keep the choice bits for itself? British Columbia, Halifax, stuff like that?