Hi OP. This sub doesn't permit hypothetical questions, since those lead to speculation. If you're interested in people's imaginings, please x-post to /r/HistoryWhatIf. However, if you're interested in knowing the actual contingency plans held by, say, the USA, then respondents are welcome to go into that here.

To answer even the slightly different question that /u/searocksandtrees posited, the answer is still pretty much, we don't know for sure.

The official American plan called for an invasion of Japan proper. That was the purpose of invading Okinawa, was to provide air bases and forward positioning for such plans. However detailed plans hadn't been finalized. MacArthur's staff was working pretty hard on getting them done. We do have a rough outline of the plans though. The first stage (Olympic) was planed for November. That would involve 14 divisions and would be aimed at gaining control of about a third of Kyushu. Once that was complete, air bases would be built to support Coronet, where 15 divisions would be used to attack central Honshu, around Tokyo. That was expected to kick off around March. Follow up operations were still completely up in the air.

Officially everyone supported an invasion. That was the agreed upon strategy and as such no one wanted to speak out. Privately there was a lot of discussion amongst the generals and admirals as to whether an invasion was desirable, or even necessary. The Americans were shocked at the losses on Okinawa and Iwo Jima, and there was extensive concern over what casualties might look like fighting the Japanese on the home islands. So people like Admiral Nimitz were pointing out how a blockade might make an invasion unnecessary. General LeMay was convinced that firebombing alone could bring Japan to surrender. Firebombing raids took place between the atomic attacks, and even after Nagasaki. The last air raids took place on the 14th, with the radio broadcast announcing the surrender on be 15th. That was without the new Air Force that has been installed in Okinawa. It was expected that Doolittle could match what LaMay was already doing and help level Japan twice as fast.

As to further nukes, there already was a third bomb on Tinian that could have been ready to go within a week. Truman had ordered that no such bomb be dropped without his explicit approval. But production was continuing. It was expected that around a half dozen or so would be ready by November. Preliminary plans were for them to be used to support the Olympic landings, probably against cities that where transportation centers inland of the landing beaches.

So those were the American plans, basically to continue to tighten the blockade, blow the everloving shit out of everything the bombers could see, and if nothing else brought the Japanese to surrender to invade in November 1945.

/u/QuickSpore outlined what was,as a whole, named "Operation Downfall". The Japanese rightly assumed that an invasion of Honshu would come up from Okinawa into Kyushu so they made appropriate plans (Operation Ketsugo) to make the invasion so daunting/costly that they could at least avoid utter defeat. Losses on both sides would've been immense. Leading the Japanese to hope that the Americans simply wouldn't pursue the option (and honestly it wasn't that attractive a prospect).

And while I'm not sure of specific plan names it's highly likely the USSR would've involved itself in some way. It would be speculation to try to say just how far they would've tried to go but North Korea sets a likely example of designs Stalin would've had in the area.