...this is more of a thought experiment than anything, but
maybe there's a Turtledove expy who writes a series of books, spanning multiple decades from the 1860s to the 1940s, where the Confederate States of America - owing to the War of Secession ending later than in TL-191 - is in a far weaker position, with their largely agrarian economy crippled by an overreliance on cotton and a tense political situation, and ultimately end up becoming dependent on outside powers: Britain, France and, most of all, the resurgent Second Mexican Empire, who end up buying parts of the Confederate West and generally making Dixie their junior partner. This is not lost on the United States, whose revanchism becomes tinted with anti-imperialism and virulent Hispanophobia (or, in more extreme cases, stringent anti-Papism). The standoff between the United States and the Second Mexican Empire dominates New World geopolitics from then on, with the two practically diametrically opposed powers battling for influence in the impoverished South, Central America with its various proposed shipping routes, the Caribbean and South America, with the Empire of Brazil falling to a pro-Mexican coup d'etat with Confederate bayonets. Of course, by the beginning of the 21st century the Empire and the United States are looking for allies elsewhere, with some European powers becoming more keen on allying with the ardently Republican power...
The unofficial title is, of course, Remember the Alamo.
I apologize if a similar proposal came up already in this thread, but I couldn't help but make one nonetheless.