What would be some crucial differences between a Germany united under Austria and a Germany united under Prussia in their government, domestic and foreign policies, military, etc? The POD is that Austria defeats Prussia in the Austro-Prussian War. Unlike in OTL, however, Austria allows Prussia to join a future German state, but Otto Von Bismarck chooses to resign and fade into obscurity, eliminating him from further involvement in German political affairs.
If that POD is too implausible, then you may choose your own.