You need to prevent OTL Congress of Vienna, which means preventing the loss of significant portions of post-Partition Poland to Russia, and also prevent them from gaining most of the Rhineland; to do that means altering the great powers diplomacy, Prussia as the weakest basically took the Rhineland because they were told to do so and because no one else wanted those lands (and the burden of guarding against France) nor France to get them. There are basically two PoDs-
1. avert the revolution, or end it quickly, preventing the massive map redrawing; or
2. make the Revolution successful, whilst preserving Prussia- best way is to get Prussia to stay out of the initial war and ally Napoleon a la Bavaria, gaining Saxony and a free hand in Poland and building a Baltic empire.
Prussia at this time was very much a composite, multi-ethnic state, one which only post-Vienna was at all interested in Germany beyond the Baltic/Pomeranian coastline. Keep Prussia focused east and out of the Rhineland in any significant capacity and they will naturally tend to evolve into an Austria-Hungary style dual monarchy stretching along the Baltic coasts, assuming they aren't defeated and carved up by Sweden/Russia/Austria of course.
Alternatively one could angle for a Brandenburg-Polish union a la Wettin Saxony, which is hardly impossible at any time in the 16th, 17th and 18th centuries.