Rekjavik
I don't think this is practical. If I recall correctly his share of Poland after the 3rd partition included a little hamlet called Warsaw. As such it had most of modern Poland. Can't see any way it regained all that region except in opposition to Russia. Which would require very good relations with most/all the other great powers. Given that I couldn't see it sitting on the Rhineland, although if the Saxons clung to Napoleon to the end he might get all of Saxony.
Similarly he might get all the Rhineland and Saxony if working with Russia and defeating the Austro-French-British alliance that blocked such gains. However to do this Prussia would have to give up at least as much of Poland as it did historically if not more.
Steve