This is for a game mod scenario. As pointed out earlier, Britain takes a very different path in the 19th century in its relations with the US and ends up losing its empire and balkanising Canada and the US, the latter losing much of the South and West. It's an ASB-lite scenario in its backstory, but I would like the scenario to be well-constructed and stylistically self-consistent to make a detailed, compelling scenario for the later period of time depicted in-game.
This topic does raise the issue of who will be the major and third parties in the North though. Does losing the South, Southwest, West Coast, and parts of the Rockies make populists/progressive/socialists more or less powerful in the North? Can those OTL third party currents coalesce into an enduring, unified third party, or even replace one of the major parties? Or will those political currents be butterflied away along with the Gilded Age without Southern Democrats?
Will the US' relations with Canada and the South be revanchist or is the US' political culture too developed for that? Will relations with the South resemble the Koreas (militarised and a desire to reunify through military conquest), India-Pakistan (militarised and a desire to remain separate), or Pakistan-Bangladesh (no point crying over spilt milk)? How would this affect the North's domestic politics? Could the Technocrats' idea of a North American Technate appeal to those who want to right the wrongs committed by the South and its allies (Canada and French puppet Mexico) during the war of secession?