From
The Final Months of the War With Japan: Signals Intelligence, U.S. Invasion Planning, and the A-Bomb Decision by
Douglas J. MacEachin, Center for the Study of Intelligence (CIA) - a discussion of Americna war planning in mid-1944:
The debate nonetheless continued through the rest of 1944 and the first few months of 1945. Admiral King, while nominally sticking to the position that the end-game would be an invasion of the Japanese homeland, advocated various operations to be undertaken between the seizure of Okinawa and the invasion of Kyushu--for example, attacks on small islands and coastal areas of Japanese-occupied China between Formosa (Taiwan) and Japan. Some analysts have postulated--plausibly--that these operations were seen by their advocates as a way of creating more time for the bomb-and-blockade campaign to produce the surrender they believed could be obtained without an invasion of the homeland.
Considerable debate also took place on the question of an amphibious assault on the northern Japanese island of Hokkaido. The strategic outline accepted by the Joint Chiefs in July 1944 had specifically named Kyushu as the site for the initial invasion. But some planners--with support from General Arnold--argued for attacking Hokkaido first.
Although these discussions initially focused on Hokkaido as an interim step between Okinawa and Kyushu, the debate evolved into an examination of Hokkaido as an alternative to Kyushu. Nearly all members of the Joint War Plans Committee (see footnote 2), however, strongly supported targeting Kyushu rather than Hokkaido. They also objected strenuously to any diversion of resources toward an interim operation.
Apparently Hokkaido was first considered as an interim step between Okinawa and Kyushu, and then instead as a substitute for Kyushu - ultimately, envisioned as a 10 division operation targeting southeastern Hokkaido, particularly around the Sapporo plain. The advantages were obvious: It was sure to be much more lightly defended than Kyushu, and more likely therefore to achieve strategic and tactical surprise. The disadvantages have already been noted: It's much farther from Allied bases, the weather was likely to be worse in the time period in question (and weather would affect not only naval operations, but also the construction of new airfields, and other infrastructure for supporting CORONET in March 1946), and an assault could not be supported by land-based air. Okinawa was, after all, largely targeted precisely because it was close enough to provide land-based air cover for an assault on southern Kyushu. For the Joint Committee, that was enough to shelve it and focus instead on Kyushu.
C-Span had a symposium discussing this as a kind of alternative history exercise, BTW, which may be of interest:
"The Hokkaido Myth: U.S., Soviet, and Japanese Plans for Invasion.” It's about an hour long. Worth watching if you have the time.
If there's a logic behind doing it anyway, it would have to be based on the premise of a true awareness of a) how heavily defended southern Kyushu (and the precise landing zones) would actually be, and b) how very difficult it would be for Japan to shift assets (especially air) back up north to meet a Hokkaido invasion, given the lack of fuel and the collapse of Japanese transportation infrastructure, especially the inter-island ferries, under American mining, submarine and air attack. Only at that point would the advantages and disadvantages of Hokkaido be in a position to be reassessed in a light favorable enough to give the operation a fighting chance for approval.
The logic at that point might be that the light opposition would more than offset the inability to support the invasion with any land-based air (and the need to ferry any such AAF assets to airfields secured in Hokkaido by sea).
Yet there's a risk here: If we can interject in to US war planners (some sudden intelligence coup, for example) the full awareness of Japanese defense capabilities in Kyushu, it's arguably more likely to turn U.S. planners against any invasion at all, in favor of the bomb-and-blockade strategy that was favored by Nimitz and King by the summer of 1945 - favored, it should be noted, precisely because their intelligence was uncovering just how intense the Japanese buildup on southern Kyushu was becoming.