Get a Prussia-Austrian-Hungarian kingdom with a POD after 1700s
It has to be fully equal so not just one or two conquering the others and forcing them into the alliance. They must also grab the other Germanic states. (Optional to get Luxemburg and Denmark).
Religion is an issue if you're trying to unite Catholic Austria with Calvinist/Lutheran Prussia by marriage.
There was a TL, I think it was called "Zwei Adler, ein Kaiser" (German for Two eagles - one Emperor) or something, where Frederik the Great married Maria Theresia of Austria after converting to Catholicism, uniting their realms and leading to a more liberal/progressive Austria, Austria having a military genius defending it and a lot more pressure to centralize the Empire.