Maybe have Kerensky hold onto power in Russia long enough for the Germans to break, and as a result Russia gets a say on the Treaty of Versailles, giving them significant influence over the Slavic territories of the Central Powers, and with East Prussia either being occupied by Russia, or under some sort of Russo-Polish protectorate.
During the interwar years government instability, ethnic tensions, and leftist insurrections result in Russia falling under fascist government after a civil war. The Nazis rise in Germany and WW2 plays out in a similar fashion, with Russia signing a non-aggression treaty with Germany so they can focus on stabilising their Empire, and the Germans eventually turning on them, and Germany ultimately being defeated by an alliance of convenience between Russia and the Allies.
The brutality of the Russian occupation is considerably worse than the Soviets IOTL, with even more violence and ethnic cleansing. When the Cold War breaks out the Russians declare their occupation zone to be a Russian protectorate, reconstituting it as the Prussian State, whilst directly annexing Poland, Czechoslovakia, and East Prussia (which are subsequently ethnically cleansed of Germans). In the Prussian State a cultural and education policy is put in place to encourage "better integration and cooperation with the Russian sphere", which involves the mandatory teaching of Russian and Cyrillic in schools and an immigration policy that encourages the settlement of Russians, and other Slavic populations. Unlike the OTL GDR, the Prussian State takes little to no action with regards to refugees fleeing to the West, even giving it tacit support with regards to Germans, which results in a large out-flowing of ethnic Germans. By 1970, although they remain the largest single group, ethnic Germans/those who speak German as a first language only make up a plurality of the population of the Prussian State, with the rest made up of Russians, Poles, Ukrainians, Jews (who have been "encouraged" to resettle in Prussia by the anti-Semitic Russian government), as well as other Slavic and Baltic minority groups. Mandatory teaching of German is eventually dropped.
Following the fall of Fascism in the 80s the prospects for a peaceful reunification of Germany, a long time goal for the Western powers, are disrupted when a referendum on reunification is narrowly defeated. Attempts to transition to democratic self-government are difficult, with attempts to craft a centrifugal Prussian identity undermined by rising ethnic tensions, which are in turn exacerbated by class tensions (despite making up less than 15% of the population ethic Russians own about 70% of the wealth and dominate government positions). In foreign policy Prussia seeks to expand its hegemony in the Baltic, entering into a union with East Prussia and the Baltic State, both of whom have large Russian populations, greatly angering non-Russian nationalists, who see this as a continuation of Russian colonialism. Ethnic tensions eventually reach a fever pitch resulting in the Baltic Wars of 1993-1999, which ultimately ends with the partitioning of Prussian-Baltic Union between Germany, Poland, Lithuania, Estonia, Latvia, and an ethnically Russian rump Prussian state centred around Danzig, and incorporating East and West Prussia, as well as parts of Lithuania and Eastern Pomerania. Thousands died, millions were displaced, and the region has still to fully recover from the devastation.