I remember watching some crappy TL thing on YouTube about an alt WWII with a facscist Mexico, but I think a fascist USA (Or at least a fascist bit of the USA minus a few states that break off) vs. Canada and Mexico with British support might work better.
Maybe playing around with a fascist South American Union between a more fascist Vargas's Brazil and Argentina could work as an antagonist? Then they could sweep through Central America and be stopped by the US and Mexico in an enormous Stalingrad-y battle? Maybe?