I'm currently writing a TNO remake, so I shouldn't be talking here, but yeah, a huge part of alternate history for a long time was shitty, delusional alternate history about a Nazi Victory. The fucking genre started its rise to popularity with Man in the High Castle for fucks sake. I agree that a big problem is starting it off with the Victory already Done, instead of explained, which I'm gonna have to do soon in my story anyway, but here's my take on what would have realistically happened had Case Blue and Barb had been aided by fucking God.
- The Soviets are pushed back greatly, the possibility of a Coup against Stalin, who really cares? Anyhow the Soviet Government would probably refocus on Perm, or maybe behind the Urals depending on how effective the German Offensives are. If it's how the Banker described, which is complete collapse, then the Soviets would internally fracture after the coup into groups still under the Red Banner, but trying to enforce their own version of Communism. Russia, at least the Red Army, is out for the count.
- Germany declares Endseig or whatever the fuck, and instantly runs into a brick wall with partisans of all different types. Like, maybe don't declare you are going to exterminate the people you just conquered in the fucking book that gave you popularity, dipshits.
- If Germany attempted to send a WP Deal or any Deal to the British and Americans, it would most likely be rejected due to a lot of things, most notably the fact that Torch was probably or already had been underway, and that America was just getting started, and that victory and vengeance for Pear Harbor and the Declarations of War was demanded by the American Public. Americans would fill the gap of manpower greatly. America had barely scratched the surface of its mobilization.
- Italy would still get dogged on, I don't think the British or Americans would really care that much about the Soviets. After all, a lot of people knew this deal with the Red Devil wasn't going to last. The only thing that would raise concern is the Eastern Front's Manpower and Equipment now refocused on the Allies, but even then, like 20% would actually get refocused as Germany now needs to do a Croatia and play Wack a Mole with partisans, just on a much grander scale.
- We could see a much bloodier Torch and Sicily Invasion, a Second Battle of Britain, invasion of Greece and the Balkans, Norway, and France would just be an absolute shitshow. But even with Russian Infrastructure, America still holds the advantage in production.
- If Germany gets desperate enough, we could see Chemical WMDs fly, and that would open fucking Pandoras' box. The British made Anthrax just in case for something like this after all, and if Germany shoots first, the justification to fucking blanket Europe in gas is there.
- With the Germany First Policy, Nukes would strike it first. Hell, if the Feds see the situation as desperate enough, they may pressure Oppenheimer and his team to develop the H-Bomb Early.
- If Germany pulls an OTL and holds out to the very end, there would be absolutely no mercy on the Allies' end, and Germany would probably cease its existence post-war.
- Early American World Order basically.
TLDR; Unconditional Surrender was the only option after America entered the war. If you change this to 'If America wasn't in the war tho' that's moving the goalpost, and an entirely different scenario.