I actually thought about a cold-war Prussia the other day; so here's my (prolly near ASB) theory
So right after the creation of the DDR, it suffers a massive revolt from disgruntled Nazis (perhaps the east is less denazified or the allies dump all those stooges on the Soviets), said revolt gains massive support from the common folk who are tired of communist rule. The masses eventually up-stage the Nazis who started and it descends into a Warsaw-Pact-wide revolt against Soviet control, with the worst disturbances coming from the DDR and Poland.
Of course, this being Stalin and all, the revolts are crushed brutally, but not quick enough to make the idea of a new Polish or German satellite state totally appealing to the USSR. So they dig into the history books and decide to merge the DDR and Poland into the Democratic People's Republic of Prussia
(Of course, this ignores the assuredly-pissed Polish population and isn't quite the criteria, but it's all I got)
Here's a flag: