I was considering the US in WWII and I believe that, had Germany mananged to capture Moscow and Stalingrad and captured Britain than the US would probably, without the nuclear bomb drop out of the war (don't get me wrong, I'm not making a post in favour of the N bomb). Had Germany managed to achieve the above there is almost no way it would have had the man power to take up and invasion of America bearing in mind the size of land that Germany would also need to garrison would also take away from its Manpower.
So my suggestion for maybe an optional victory condition for the Axis is the complete removal of Allied countries and forces from Europe. Considering this the US would have most likely, in my opinion abandoned Europe to its fate as it had no more allies there and also had the war with Japan. Also it would be harder for it to actually invade as the Germans would have only one front to fight on and the Americans would find it very hard to supply its forces and with no friendly intelligence that was supplied by Britain
So maybe an optional victory condition should be the destruction of all Allied forces in Europe and this would cause America to loose heart and drop out of the war, though not surrender.
Also as no US cities ever got bombed, maybe the bombing some of their cities could also cause the civillians to loose heart and start to force the government to drop out. Bearing in mind that no US city had ever been bombed and the people of America had always been far removed from the suffering in Europe.
Perhaps a better victory condition would be the destruction of all Allied forces in Europe and the destruction of the US's Atlantic Navy. This would almost certainly cause the US to drop out or seek a diplomatic soloution as they had by then failed all their objectives. Or maybe the act of German soldiers landing on America after the above destruction could cause America to seek a diplomatic end to the war in return for the land captured by Germany? :confused:
I think that this is worth considering as an optional condition, especially if the simulation is to be realistic as Germany could probably have never successfully invaded the US.
Please excuse any spelling differences as I'm using Australian grammatical rules