An interesting alternate history option for the game to allow would be for Joe Kennedy to return to the USA and beat Franklin Roosevelt for the Presidency in 1940.
There were numerous political forces in the USA which were very anti-British and pro-German. Charles Lindbergh and the America First group should be noted. Most Americans did not want to die for the Imperial powers of Britain and France. Fulton Lewis, Junior, was a noted radio commentator at the time with a huge audience and favored such an outcome. The Chicago Tribune headed by William Randolph Hearst tended in that direction as well.
If Joe Kennedy had won as President in 1940, its even possible that the US would have entered the war on Germany's side and backed Japan in their bid to end colonialism and communism in Asia.
In fact during the late 1920's under President Herbert Hoover War Plan Red was developed for the US to occupy and annex Canada and ally with Japan to seize British possessions in the Pacific in a joint operation.
I'm going to have to disagree with this. Popular sentiment was very much pro-British and anti-German; Hitler's moves had alienated just about everyone. The difference was that most of the America Firsters were pro-neutrality, which is very different. Note that historically, the actual Republican nominee in 1940 was an active interventionist. Nobody wanted to enter the war on Germany's side; even the most sympathetic basically wanted to sit back and fortify the Western hemisphere while Eurasia burned.
Japan and China was a very different kettle of fish. Even most isolationists were more willing to take a hard line with Japan. Remember that the US helped lead international condemnation of the Japanese invasion of Manchuria back during the Hoover administration, so it was very much a bipartisan concern.
There were several reasons for this:
a) There was a very large and active lobby in favor of China (consisting of a mixture of business interests, humanitarians, and missionaries, for whom China had been the go to place for mission work for decades).
b) Preexisting US treaty commitments (unlike Europe, where the shenanigans around the Treaty of Versailles meant that Hitler generally wasn't violating the terms of treaties with the USA, America was a key signatory to many of the treaties guaranteeing an Open Door Policy in China and guaranteeing Chinese independence).
c) The US was concerned about possible threats to its overseas possessions in Asia, especially the Philippines.
d) Racism ("There's no way those Asians could possibly be a match for our boys! We'd beat them and be home for Christmas. Not like those superior Europeans, that would be bloody").
Yes, the US had a War Plan Red, but it was one of a large number of different contingency War Plans, not a statement of American goals (the US today supposedly has a plan for suppressing an attempted coup by the Girl Scouts, that doesn't mean that one is likely).