What changes would you make to Timeline-191/Southern Victory?
1. Different casus belli for the Second Mexican War. (I can't see Mexico being willing to sell off two of its states like that, and the Confederates trying to build a transcontinental railroad across Sonora and Chihuahua also seems really implausible. Such a railroad would be incredibly difficult giving the terrain and hostile local population, and I don't see how it could ever be profitable given that Confederate trade with the Pacific was negligible.) As a possible alternative maybe Blaine, upon assuming the presidency, has the US back a military coup in Mexico that sees Porfirio Diaz overthrow the Imperial Mexican government. Longstreet, not willing to see a pro-US government take power in Mexico (which would mean the CSA is surrounded), dispatches an expeditionary force to Mexico to restore the Imperial government. Blaine responds with an ultimatum insisting that the Confederates withdraw their troops from Mexico. Longstreet refuses and the war goes from there.
2. In the First Great War Canada falls by mid-1916. It didn't really make sense that they held out for three years given how badly outnumbered Canada was against the United States, and Canada surrendering in 1916 would help explain how TR was able to win reelection that year and why the Confederates were desperate enough that they were willing to start arming their black population (since they would realize that the defeat of Canada is going to free up hundreds of thousands of additional veteran US troops to come against them.)
3. The Socialists would have been much more successful in getting their domestic agenda enacted. It made no sense that the Socialists, controlling the presidency and both houses of Congress, would let the Democrats filibuster their old age pension and unemployment insurance bills for over a decade. (Realistically the Socialist majority would have abolished the fillibuster by 1923 or 1925 at the absolute latest if the Democrats kept using it to block the Socialist agenda.) Old age pensions, unemployment insurance, farm price supports, public housing, and nationalization of at least some industries (the power companies and maybe the railroads) thus all would have happened during the Sinclair Administration. This would probably soften the impact of the Great Depression.
4. The Pacific War would have been a much bigger deal. (After two whole generations growing up with the Remembrance Ideology, I can't see the US settling for a status quo ante bellum peace.) At a minimum the US would have fought on until they had taken the Philippines. (They need to take something big off Japan, so the American people will l feel like they won, and so Japan will know it was beaten.) This would also set up US vulnerabilty to Featherston being not from appeasement and weakness but from imperial overreach. (And especially since most of the defense budget in the early 30s would have had to go to the Navy and Marines for the fight against Japan.)
5. Agree with Lalli that there shouldn't be any holocaust parallel. It's already stretching plausibility to the breaking point to have the Confederates (a country with 1/3 the population of the US) go toe to toe with the United States for three full books. Having them holding off the US for so long while they are simultaneously trying to exterminate 1/3 of their own population is just ludicrous. (And it's not like you need to have the Confederates go full Nazi to show that they are thoroughly evil. Having Featherston impose an apartheid regime would be much more plausible while still showing the CSA to be a horrific dystopia.)
6. Also agree with Lalli about switching up the alliances from the First Great War. Maybe the Kerensky government survives in Russia. (In TL-191 it should have been obvious by 1917 that the Entente was going to lose the ar, and there certainly wouldn't have been any loans coming to Russia from the United States government, so there's really no logical reason in TL-191 for Kerensky to decide to keep fighting , and if he does have Russia pull out of the war then it might be possible to avoid the Bolshevik coup since IOTL it was Kerensky's insistence on continuing the fight that destroyed all support for his government.) If the Kerensky government does survive then the 1920s could see the US draw close to Russia since both nations are under left wing democratic governments and share a mutual enemy in Japan. Germany would fear Russian revanchism (a democratic Russia is still going to be upset about all the territory it surrendered to Germany in the FGW), so as the US draws closer to Russia that might lead to German reproachment with the British. And thus we might eventually end up with an alliance system of the US, Russia, France (and maybe China) facing off against the CS, Germany, Austro-Hungary, Britain, and Japan. (Which I think would make for a much more even conflict than what we had in the canon storyline.)
I also think the reason for the British entering the Second Mexican War was very implausible. (I can't see the Confederates being willing to give up slavery so easily, and I can't see the British being willing to enter the war on just a promise of emancipation.) Unfortunately, I can't really think of a plausible reason for why Britain would enter that particular conflict, and it's kind of essential to the storyline that they do so since without British intervention the US will win the Second Mexican War and that pretty much short circuits the entire storyline. Thus if anyone has any ideas for a more plausible justification for British intervention, I would certainly be interested in hearing them.