What If? is the name of a really good restaurant in Hershey, and I urge you to try it, but I've got something else in mind.
Every once in a while, somebody writes a novel positing a different turn that could have been taken by history; say, if Stonewall Jackson had lived and been at Gettysburg. That leads me to an even bigger "W. I.?": had the nation let the South go instead of fighting the Civil War, what might the good and bad results have been ? The income of the federal government before the fateful date in 1861 was largely from excise duties and customs fees collected in the busy Southern ports (so you can glimpse the real reason for red-hot regional issues), and Northern businessmen didn't want to be taxed to make up for that loss. Corporate America got its initial big boost from the war effort, and would possibly not have catapulted into frantic boom-and-bust expansion without it. Whether the share-cropping underclass existence of the eventually freed slaves would have come about under a Confederate nation much later in the 19th century is unknown, but seems likely. Same result with less destruction and rancor.
Given the equally aggressive spirit of the Scots-Irish working class and the Cavalier landowning upper class of a hypothetical Confederate nation, a troublemaking foreign policy toward Mexico, Cuba and the Caribbean would probably have been pursued (maybe short of European intervention, but hotheads could have pushed it to that). Would cooperation between the U.S. and C.S.A. during the Spanish-American war have led to alliances in World Wars I & II (like the Commonwealth coming to the aid of the United Kingdom)? That seems likely, and might have led to an amicable co-existence in the 20th century.
Another choice to go to war comes to mind: what would have been different if we had not gotten involved in Vietnam? Well, nothing, probably. I'm sure Thailand would have fiercely resisted any Vietnamese penetration beyond the old French Indochina (with allied help), since they had never been under European colonization and had an entirely different mindset than the Vietnamese, who demonstrated their own limits by pushing into Cambodia and battling China briefly. They were wise enough to back away from both, and return to misgoverning their own domain.
What if mankind were all like the Japanese, and didn't like the taste of sweets? Sugar, like spices, gradually spread downward and outward through populations from a desired item affordable only to the elite, from its introduction to Europe around 1100 A.D., to all of the mass market (1900). Slavery, first of the natives then of Africans in the New World, might not have been so successful and long-lasting if sugar cane had not been cultivated. At the end of the Seven Years' War (French and Indian War), King Louis was happy enough to give up Canada ("a few acres of snow," in his opinion) in order to get back Gaudeloupe from the British -- the gold mine of sugar plantations. We forget that French support for the American Revolution, and its naval expedition that secured the American victory, was in a large part based on their strategy concerning the Caribbean sugar islands. Sugar was as central to capital in Britain, France and the Netherlands in the three and a half centuries following Columbus as industry is now (Canada just cost old Louis a lot of money, and its fur trade was declining from dwindling supply and fierce competition from the Hudson's Bay Company, anyway).
Which brings up a related idea: what if we hadn't had a revolution, but had evolved to independence gradually like Canada and Australia? We'd still have been dragged into the world wars, but would otherwise have been at peace and would have health care for everybody, and the Queen is a pretty good old gal.