Huh, I was actually on the verge of posting a thread with the same question.
First of all I believe Poland would still be given southern Ostprussen as compensation for losing Eastern Poland. And Memel would still be given “back” to the Lithuanian SSR.
For one, it means the DDR is somewhat stronger in the long term, due to more territory and population. While there would be no resettlement, there would likely be very large costs to reconstruct the region, though I suspect the Soviets would be more intent on aiding the efforts. The Soviets would probably also open a naval base in Königsberg, as this in OTL is currently the main base of the Soviet Baltic Fleet.
In economic prospects it would add more land for manufacturing and agriculture, and also could be a place to send dissidents. Likewise, some of DDR’s “brain drain” would be prevented as potential defectors living in East Prussia would find it much harder to leave the country.
In general it would not change that much. A more independent Prussian identity and sense of nationalism is bound to develop from being isolated from the rest of Germany. However, it would still join a newly reunified Germany once the communist regime falls, although there is the potential that some SED hardliners could try to hold out in the enclave for several months before finally backing down.
As for South Sakhalin/Kurils, it has too small of a population, and probably not enough resources, to be viable as an independent state. I don’t think the Russian population there, which was equal or larger to the Japanese settler community, would take too kindly to being part of “North Japan”. The only way this would work is if it also included Hokkaidō.