The invasion of Japan by Allied forces was already planned, along with estimates of casualties, etc. "Operation Downfall" was the name of the plan. Two landing were planned on the Japanese mainland - one in November 1945 and another in March 1946. It was expected to involve eventually 3.5 million allied combat and support personnel.
Japan in turn was preparing for the invasion, Ketsu-Go. They had been preparing since 1944. They actually had no shortage of suicide aircraft, thousand of cheap planes, essentially flying bombs. Their plan was to launch massive kamakaze aircraft attacks (from hidden airstrips) at allied vessels to smash the invasion fleet. They estimated they could attack and damage 800 vessels in one strike. If a landing was achieved, the first one in November was aimed at Kyushu, Japan had some 800,000 soldiers to fight. These aren't woman and children, but hard core fanatical soldiers. Organized divisions, tank brigades. etc. They had already stockpiled supplies and ammo. Beyond the beaches, Japan is rocky and mountanous, a natural defendable fort. One giant Iwa Jima was exected - caves, bunkers, underground supply depots, fortified positions with machine gun nests and artillary covering square miles.
And then, as was stated, every civilian was expected to fight to the death. 28 million fanatics to the emporer equiped with ancient rifles and satchel charges.
Casualties on both sides, needless to say, were expected to be horrendous. Estimates - 1,000 Japanese and American battle deaths an hour in the first few days, up to a million allied casualties total. No telling how many Japanese would have died in battles and in mass suicides - 4 or 5 million is not out of the question.
During all this, the USSR would have been playing some games, just as they seized Manchuria during the final days of the war. Expect then to have gobbled up portions lightly defended China and Northern Japan, as well as Manchuria. Possibly we would have seen a north and south Japan divided as we say in east and west Germany.
Not to make this political, but it's absolutely incredible that someone can look a the dropping of the atomic bomb as a horrible and uneccessary act. It saved millions of lives on both sides by ending a terrible war.