Coring in 12 years (the timespan of the game) in the 20th century (when very, very few nations have successfully "cored" areas, even when given far longer time periods to do it - the 20th and 21st centuries have tended towards fragmentation of states rather than nations becoming larger) feels too rare to me (ie, just my opinion) to be worth a mechanic. I can't think of a single successful example of what we'd think of as coring in the 1936-48 timeframe. If you can think of something, then post it up and it might be a useful case study for developing the mechanic around.
At the end of the war, Germans were expelled from East Prussia, Silesia, East Pomerania and parts of Czechoslovakia. The USSR and Poland split East Prussia between them, Poland took control of Silesia and East Pomerania etc. You could consider this to be a form of "coring" - the original German populations were removed, other new populations moved in. This is probably the closest thing I can think of to "coring" - in the sense that the territory had very few or no people left to resist the new government, and the new populations were loyal to their homeland. The USSR and Poland also had a population exchange for Poles, Ukrainians and Belorussians who were on the "wrong side" of the Curzon line (the new dividing line between Poland and the USSR, after East Poland was conquered by the USSR in 1939).
The territory gained by the Belorussian and Ukranian SSRs remain the territory of their successor states today. Kaliningrad Oblast is a territory of the Russian Federation. The border between Poland and Germany is not controvertial, and remains what it was at the end of WWII (with the exception that East and West Germany unified, obviously). I believe there were lesser expulsions and exchanges in Hungary, the Baltic States, Romania, the Netherlands and so on. The modern states of Europe are HIGHLY homogeneous compared to their Pre-WWI, and even Pre-WWII states.
The problem, of course, is that HoI4 is not going to model ethnic cleansing, even if it is in a "gentle" form, i.e. population exchange (which is still highly distressing and undesirable to the families who get forcibly relocated and lose their homes). And it is CERTAINLY not going to model what the Germans intended to do if they were victorious in the east.