What impact would the Allies deciding that Hokkaido and Ryukyu must have their independence restored, and thus must be confiscated from Imperial Japan, after WW2 have? I think an independent Hokkaido + Sakhalin (i.e. an Ainu state) and an independent Ryukyu state would have major impacts on postwar Japan.
Okinawa/Ryukyu islands were confiscated from Japan after WWII anyway, and kept under US suzerainty until 1971 IOTL.
Hokkaido was a close call IOTL - Japanese removed nearly all armed forces from island at the end of WWII, leaving only symbolic coastal patrols, and Soviets were arranging a probing attack at Rumoi. It was scrapped after naval battle under murky circumstances, when spetsnaz-carrying submarines have run by accident into convoy of Japanese refugees from Sakhalin at the final approach to Rumoi. The submarine commanders reported convoy as troop transports, which led Soviet leadership to believe the Hokkaido still have significant infantry to protect beaches, and the invasion plans were scrapped. Slightly different convoy timing may result in capture of Hokkaido by Soviets.
From economics standpoint, not much effect is expected. Both Okinawa and Hokkaido are backwater for Japan, Hokkaido mining industry was useful but not essential (it struggled to be competitive against imports since removal of price controls in 1963 anyway). Hokkaido, even if captured by Soviets, will be reintegrated into Japan proper around 1988, as happened with East Germany.
Main impact of temporarily Soviet Hokkaido would be ruined infrastructure of Hokkaido. After reversion to Japan, we likely to see large part of island converted to nature preserve. Also, larger Japanese population flux to Honshu/Shikoku/Kyushu. (about 1.5 million additional persons). I assume here all Japanese are forcibly deported, as Soviets did with Sakhalin or Kuriles. Not going to alter Japanese demography significantly due to to much larger fluxes of Japanese evicted from Taiwan, Manchuria and especially Korea.