Short answers, pulled from the story.
Operation Downfall was the Allied plan for the invasion of Japan's home islands near the end of World War II, consisting of two phases: Operation Olympic targeting Kyushu in November 1945 and Operation Coronet targeting the Kanto Plain in spring 1946. It was canceled when Japan surrendered following the atomic bombings of Hiroshima and Nagasaki and the Soviet declaration of war on Japan, which triggered the Soviet invasion of Manchuria.
Casualty projections varied widely depending on scope and source. American losses were estimated at anywhere from roughly 220,000 to over a million, with the Army Service Forces January 1945 document projecting roughly 863,000 Army dead and wounded through early 1947, not counting Navy and Marine Corps. Japanese military and civilian casualties were projected in the millions, with physicist William B. Shockley estimating at least 5 to 10 million Japanese deaths in a July 1945 worst-case document.
Operation Olympic was the first phase of Operation Downfall, targeting the southern third of Kyushu on the 1st of November 1945, with 14 U.S. divisions in the initial landings. Operation Coronet was the second phase, targeting the Kanto Plain south of Tokyo on the 1st of March 1946, with up to 45 U.S. divisions assigned for the initial landing and follow-up, a force larger than the entire Normandy invasion.
Japan's defensive plan was called Operation Ketsugu, meaning roughly "Decisive" or "Final Battle." It called for committing the entire population of Japan to resistance, backed by a propaganda campaign titled "The Glorious Death of One Hundred Million." By August 1945, Japan had over 735,000 military personnel deploying on Kyushu alone, plus a Volunteer Fighting Corps of 28 million civilians including men aged 15-60 and women aged 17-40.
By war's end, Japan possessed roughly 12,700 aircraft in the home islands, approximately half configured for kamikaze attack. Ketsu plans for Kyushu envisioned committing nearly 9,000 aircraft. The U.S. Strategic Bombing Survey estimated that 5,000 kamikaze sorties could have sunk around 90 ships and damaged another 900, roughly triple the Navy's losses at Okinawa.
Both were actively studied. President Truman vetoed chemical weapon use against personnel in June 1945, though crop destruction remained under consideration. On the nuclear side, at least seven Fat Man-type plutonium bombs were projected to be available by X-Day, and Manhattan Engineer District head Ken Nichols wrote that roughly fifteen atomic bombs might have supported the invasion. The ingredients of a crop-killing compound tested for Downfall, designated LN-8, were later used to create Agent Orange.