True, we are playing with Stalin's mind.
For Stalin to see anything positive about anything German will be difficult.
Remember what happend to the Volga republic? and that was based on the ethnic make-up, not actions. And even some 4 generations later.
Ivan
Let's play again with Stalin's mind!
Stalin decides that East Prussians are not, after all, Germans. They were merely Germanized (and brainwashed by capitalist values) Prussians, and therefore were to be re-Prussianized. The part of East Prussia subjected to the 1920 LoN referendum is unilaterally given to Poland, and ethnic Germans are moved north. East Prussia is combined with Danzig, again to control Poland's external trade. Because Prussians were closely related to Lithuanians, Stalin makes East Prussia the Ostpreußischen Autonome Sozialistische Sowjetrepublik within the Lithuanian SSR. As much as possible, the link between East Prussia and Germany is cut off through Soviet propaganda.
Also, ethnic Germans throughout the USSR are expelled according to reliability and utility. The worst troublemakers are expelled to West Germany. The more useful and reliable Germans are expelled to the DDR. The rest are resettled to the OPASSR.
When the USSR collapses, Lithuania takes East Prussia with it. Decades of Soviet propaganda had convinced East Prussians not to join Germany, yet they're now a suspicious minority in a new land. They're also leery of seceding from Lithuania due to Hitler's shadow, and must accommodate the 30% Russian community concentrated in Konigsberg. Fun times.