What was the planned second phase of Operation Downfall's invasion of Japan called?

The planned second phase of Operation Downfall was called Operation Coronet.

Operation Downfall was the proposed Allied invasion of Japan's home islands. Its first phase, Operation Olympic, was scheduled to attack southern Kyushu in November 1945. Operation Coronet would have followed in spring 1946 with an invasion of the Kantō Plain near Tokyo.

The plan was never carried out because Japan surrendered in August 1945. The atomic bombings of Hiroshima and Nagasaki, the Soviet declaration of war, and the Soviet invasion of Japanese-held territory all contributed to the rapidly changing strategic situation.

Operation Coronet is sometimes mistaken for the entire invasion plan, but Downfall was the overall operation. Olympic and Coronet were its two component operations. Had the plan proceeded, Coronet would have involved a huge concentration of Allied forces against Japan's most densely populated region.