One that I’m pondering is , “What if the War of 1812 went very badly wrong for the USA?” No British reconquest involved, Europe & India were quite enough. Perhaps the British began arming and agitating the tribes south of Canada to set up a buffer zone between a restricted USA and Canada. If this inhibited westward expansion and industrial development in the north there may have been no Civil War. A USA dominated by the South may have been less attractive to Roman Catholic migrants. These may have headed to the Spanish & Mexican territories. That shift in manpower on its own would have radically altered history. Imagine Hollywood making Spanish language movies (It’s still in Mexico) about the brave Irish helping defend the republic.