I'd been thinking of posting a WI Pearl Harbor December 7th 1940 scenario, so its a coincidence that someone else did it just yesterday!
For me the interesting WI, is what if Stalin's WW2 plan had worked out? Stalin had expected WW2 to play out like WW1 and planned to keep the USSR while the capitalist west destroyed itself. He would then step in to take the spoils hopefully in conjunction with a post-war revolutionary wave.
I think its an interesting military question of whether the Western allies could launch an amphibious assault on the european continent against the full-weight of the nazi army. The US army would have to grow to Soviet-sized proportions. If Japan attacks PH in 1940, Hitler will declare war on the USA as in OTL. If anything the case for going to war with the USA is much stronger for Hitler in 1940 when the UK stands alone, than in 1941 when his Moscow attack is faltering. Hitler launches a massive Middle Eastern offensive in mid-1941 following his Balkans campaign, in order to finish off the UK before the US can intervene in sufficient numbers. West Africa provides the closest possible geographical location for a buildup of US ground forces. US industrial might should be enough to eventually secure the southern Mediterranean and air superiority over Europe. Since in this timeline Turkey is either occupied or an Axis, one possible invasion route is across the Dardanelles into the Balkans. The war might eventually have to be resolved by atomic weapons. In OTL the US army reached 15 million by 1945, but was never fully engaged.
Another interesting question is how much does Stalin's best case scenario actually benefit the USSR? Soviet industry is in much better shape having not been devastated by the war. But the military, while not devastated by the horrific casualties, is less combat-hardened and resembles more the army of 1940 that could hardly handle Finland rather than the victorious army in Berlin. The Western allies are far less sympathetic to a USSR that sat the war out and are unlikely to accept Soviet moves into E.Europe as easily. Perhaps in 1945-6 the Soviets, at allied request, conduct an August Storm style campaign in Eastern Europe against already exhausted Nazi forces. The other thing is that Communist partisans are far less likely to play as leading roles in the resistance movements without the blessing of the comintern. Thus the European Communist parties enjoy far less prestige than they do in OTL.