Hmm. Let's say Zuikaku is not spent at Leyte. Instead she is repaired at Yokohama and could also escape the Kure bombings. Knowing about the planned invasion the Japanese command decided to keep a force in reserve, either to be used as kamikaze unit or to have at least a basic force for defending the islands after the war. So they base this small fleet, Zuikaku, Nagato, an Unryu class carrier, some cruisers and destroyers in Hokkaido.
Then nukes and Soviet declaration of war. Stalin tries to get the Kurils and Hokkaido. With 30 US built landing craft the Soviets try to invade the Kurils, not aware, that the rests of the Japanese fleet were there (perhaps the US did not tell them, perhaps it was indeed a secret for them as well). Anyway, the Soviet marines could land, but eventually the Japanese fleet appears and sinks all Soviet ships. The few destroyers of the Pacific fleet make a daring attack but were sunk as well. However, some Soviet subs manage to hit all three capital ships. However, all were set aground in shallow waters and could be used as coastal batteries resp. air fields.
Soon after the Japanese surrender. Stalin is furious and demands the Kurils. He is said, he can have them, if he can take them until the US came. As he has no means to do so, he does no attempt, knowing too well, that the Japanese would fight, regardless if there were a surrender.
Shortly after the US forces arrived there and the local commander surrendered. As the US feared about a new Soviet attack, they take over the three capital ships. USS Nagato, USS Zuikaku and USS Unryu. They want to use them only until own defense systems arrived. Then the ships should be used as nuclear bomb targets or being scrapped. But eventually they were not put out of duty until after the Bikini bomb trials, and after they were half forgotten.
In 1950 the USN tried to find a scrap metal buyer for them, but a Japanese petition was made to preserve them as museum ships, a petition, which is joined by US Admirals like Nimitz. Only little later the Korea war breaks out and Japan could rearm. "For the defense of the Kuril islands" the ships were given back to Japan, after being repaired. They were only allowed to be used between 40° and 60° N and 150° and 160°E, "in case of emergency" (or with US okay) in Japanese Home waters.
In the 1960s the ships are too old and become finally museum ships. They are known as the three "Defenders of the North", "Stalin's nightmare" or "Saviours of the Kurils".
Adler