Operation Downfall had two parts: Operation Olympic, which was the planned invasion of southern Kyushu, and Olympic, which was the planned invasion of the Kanto Plain.
Over the summer, casualty projections and estimates of Japanese military strength became increasingly pessimistic. On Kyushu, the Japanese had amassed nearly a million soldiers and had stockpiled thousands of kamikaze planes.
Is it possible for the US to decide invading Kyushu will be too costly, and to instead just invade the Kanto Plain, either in the fall of 1945 or spring of 1946? Given that the Inland Sea was extensively mined and Japan's rail network would have been intensely bombed, it would have been extremely difficult to transfer the Kyushu garrisons to Honshu, and they could be left to whither on the vine.
However, I think there are some drawbacks. The Coronet invasion force might still pass close enough to Kyushu to be attacked by kamikazes, and occupying southern Kyushu would allow the US to build airfields closer to the planned Coronet landing sites (though planes launched from Iwo would still be able to reach Japan).
Also, as an alternative, did anyone consider landings on Shikoku?