The allies losing ww2 or atleast the war ending in a state where germany is a superpower has to ask the questions first of what the US does during the war, and by extension japan. If hypothetically FDR was regular American isolationist and refused to provide aid to great britain before being attacked first then the brits would have a tougher time during the war, Japan is the biggest factor here. You could argue that if FDR wasnt clearly highly interventionalist against the axis the japanese may not have attacked but even that is highly debateable, the empire of japan was incredibly bloodthirsty and would have inevitably conflicted with American interests in the pacific. In my opinion even if the US stayed fully isolationalist then japan still would have probably attacked.
Assuming japan doesnt fuck everything up for the axis and the US stays out of the war and doesnt provide aid to the british or the soviet union then the british may be forced to sit down at the negotiation table with germany, churchill wasnt going to give up easy but without any help and no outside economic assistance the brits eventually would have started to buckle. Combine that with north africa falling into axis hands and things start looking pretty good for germany. A full scale invasion of the island however would be nearly impossible, the amount of ships and resources needed would be astronomically high, resources the axis couldn't spare. Not having to direct the obscene amount of resources to the kriegsmarine to halt supplies getting to britain and the rest of the western front not needing a massive garrison, the germans would have a better time on the eastern front, the soviets eventually would have gotten their shit together but its possible stalingrad and by extension the soviet oil fields would have fallen into german hands, impairing the soviet economy. The counter offensive wouldn't be as strong and its possible battle lines could have solidified instead of the rush we got in history. Whether an armistice is reached or not is in question. Total control of all of russia is far fetched due to how effective scorched earth was and the already massively overextended german supply lines
No matter what happens the axis surviving as strong entities means massive civilian casualties. The slavs in eastern europe would suffer immensely at the hands of the germans, potentially millions of not tens of millions of deaths. The chinese would also suffer greatly under the japanese, im unsure if the death toll would exceed that of mao's autism but it would certainly be close. Italy likely starts losing relevance until being relegated to the status of germany's bitch, assuming that doesnt happen during the war like it did in our timeline.
Germany likely becomes a super power or something close to it, britain is absolutely weaker and its empire shattered, the soviet union becomes an even bigger shithole assuming it doesnt collapse internally, the japanese form a massive empire spanning much of south east asia, potentially a step below a super power. Communism likely becomes a dead ideology, the soviet union getting merked and no maoist china means it gets relegated to obscure circles and the best it could hope for is becoming popular in a minor backwater nation, facsism and its derivities become the new hot thing.
The US is a big question, its a big assumption the US stays fully isolantionalist with all this happening around them, no matter what internal changes similar to how the US changed during the cold war are likely to happen. I cant even predict what happens to latin america Most of the predictions I made past the first paragraph may aswell be fan fiction with how big the changes are that I established in the first few sentences.
