All of West Prussia
East Prussia south of Konigsburg
Silesia north and east of the Oder
This puts the Polish border both farther west and farther north than the Second Republic's borders in OTL. And you know what? That's fine. Both Germany and Poland were conquered enemies, and both are to be integrated into the Syndintern. The important consideration is not historical or even ethnic borders: the important consideration is what the Syndicalist movements of these territories will be able to govern once entrusted with power. In the situation as it's developed, with a German syndicalist movement emerging from pre-war repression and a wartime state of seige, it makes absolute sense to give as much territory as possible to Poland in the short term. Future border adjustments can be worked out between the governments of Germany and Poland once those governments are both securely syndicalist, with the fraternal oversight/mediation of France, Britain, Italy, and the rest of the Syndintern.
Also, for the record, all support to Italian rule over Istria. Natural boundaries (such as the water divide on which DH draws the eastern border of Istria) deserve great consideration inasmuch as they help draw a territory into an economically united unit. Not to mention it looks nicer on the map.