I am in general agreement with several of the posts here. Current Mexico, the Louisiana Territory, the Floridas, the Philippines, Cuba, Puerto Rico, parts of Central America, parts of OTL Western Canada. Additionally, I could see, after a POD where Spain keeps Louisiana and Mexico becomes independent, a war with the United States. If Mexico wins, then they might take some territory off the US, perhaps the Old Southwest south of the Tennessee River and southeastern Georgia, as well as some frontage on the western Great Lakes. This Mega-Mexico would likely have the resources and population base to make it one of, if not the, most powerful nations in the world.