I find the idea of East Prussia with or without Danzig becoming a SSR a very interesting idea that could work.
Firstly the only person who matters on if it is created is Stalin. The reasons he could decide to do it would to be for the following reasons:
1: As it is viewed as the centre of Prussian militarism where better to show the world the power of the revolution that it can change militaristic Germans into staunch communists.
2: It would be a neat solution to the issue of ethnic Germans in the USSR which totaled around 1,400,000.
3: It would be a snub to Poland especially if Danzig is not given to Poland as it would make Poland largely reliant on Moscow's good will for accesses to the sea via. (I think depending on ports in Pomerania which I am not to sure on.)
4: It would also expand the USSR further west. This would not really mean much as everything up to West Germany is in Stalin's pocket, but it would be an egotistical thing for Stalin.
5: There are going to be some other reasons that I have probably forgot so feel free to add some more.
Just some things about the population of the proposed SSR. None of the original population would remain because the area would be mostly depopulated by war but also because Stalin would want the Germans to be already partially Russified which Germans from the USSR would be. I would say that the population would not be supplemented with Germans who were from land Germany's lost eastern territory or from POW and would come only from the USSR and possibly one or two other Eastern European states such as the 700,000 from Romania.