I would not give Posen back to Germany as it was majority and historically Polish, but the rest of the Eastern Lands you must agree on. Königsberg was founded in 1255 by the German Teutonic Order. In fact, many have called Kaliningrad “an error in history” due to the large German culture from East Prussia. If these lands are restored to German rule, you will be helping Germany restore a 700-year-old country that was abolished by the Allies illegally in 1945.
Why should the German People be punished for something that a minority of their population did? That land had been theirs since the time of Rome thus not giving the argument that it was "reclaimed Polish land" a foot to stand on. Repetitions should have been paid to the countries damages but attempted ethnic cleansing does not excuse ethnic cleansing.
I believe that the only way to fully reunify Germany is to return its former territories owned by the Kingdom and Free State of Prussia during the North German Federation, German Empire and Weimar Republic. The state should be reunited with its former territories for the reunification to be complete.
The germans have lived there for centuries. The potsdam conference cut so deep into german territory that the people who lived in now Western Poland have never been polish. There have been for centuries german. This also brings the facts that the oder neisse line was agreed because the soviets have expelled poles from pre-1939 eastern poland ( what is now western belarus and ukrainian ruthenia). We all know that a lot of germans were expelled that to this day hold resentment and this still exists in germany in the form of fringe groups that advocate revanchist ideas against poland due to the fact that this is a critical importance to the dignity of german people. In line with this, poland should be able to reincorporate ruthenia, galicia and western belaruds into its territory so as to give back polish lands impuned by soviet threats and imperialism. Belarus should also be able to be able as well to incorporate sizable russia territory.
Note: only silesia and pomerania are referred in this. East prussia (ostpreußen) should be polish and that includes the whole Kaliningrad oblast.
After WWI Germany was infected with a disease known as Adolf Hitler the Third Reich. When the Prussian people were inoculated of this sickness after WWII they thought they were free, but on the contrary. With the militaristic expansion of the Soviet Union, they had effectively taken all of the Kingdom of Prussia. In 1947, the Kingdom was outlawed due to its supposed role as the fuel for the Nazi Empire. But to relate the peaceful and socially enlightened Kingdom to the rancid and repulsive Nazi Regime is an utter perversion of history and an overlooked atrocity of the allied ministers at the end of the war. If Hitler was a reincarnation of Friedrich the Great then Joseph Stalin was a daisy-picking unicorn lover. The neglect to return the former regions of the Kingdom of Prussia to Germany was a crime in and of it's own. During the so-called "German Reunification" millions of ethnic Germans were forced from there homes with no recompense. With the collapse of the USSR and especially the Polish People's Republic, the German–Polish Border Treaty of 1990 has been rendered inert. Germany should be rebuilt entirely so that he can finally be cured of his recent dark ages, maybe even by making it a constitutional monarchy as the new Kingdom of Prussia under Prince (King) Georg Friedrich in order to revitalize German culture.
Saying that these lands should be returned is like saying Italy should get Ethiopia back because it owned it for a while or that Austria should get Hungary for the same reason. Also the high German population can be attributed to the German (Nazi) murder, relocation and cleansing of the land of native cultures.
It should be up to the people by referendum, but you will always have others who think the referendum is a sham. If going off historic maps, then America should cede the Midwest to France who must then cede it to the Native Americans, and so forth. When should we start the historical maps? Japan owned a huge chunk of China during WW2. Should that be given back to japan?