While DeathDemon has a very good point about firebombing, it would be interesting if the Japanese surrendered in time to prevent the split between the Democratic People's Republic of Japan in the north and the Republic of Japan in the south. We might even see the Emperor survive as a British-style constitutional monarch, though given the attitude of the populace of the Allies towards him that's probably ASB.
I wonder, if nuclear weapons had been used in 1945, do you think the Japanese War (assuming it still happens) would still go nuclear, or would the world have been horrified enough by an earlier use of nukes to prevent the US from nuking the North Japanese army when they invaded South Japan?