One thing to keep in mind is why Austria and Prussia were able to grow to be such powerhouses in Germany: they both had territories outside the HRE that were not influenced by HRE politics and that they could rely on for manpower and support. So to have a third (or in this case in Prussia's absence, a second) big German state, you need one that can lean on non-HRE territories to draw some power from that they can swing around. Some suggestions:
- Sweden that rules key German territories, such as Brandenburg
- A state that rules the Netherlands as well as territories in the Rhineland (Hanover?)
- Bavaria that holds some of Northern Italy
So did the rulers of Saxony and Hannover.
It did not make much of a difference.
Austria and Prussia surpassed others thanks to a combination of a fairly effective expansion, consolidation & influence policy within the Empire.
An example of how latter's expansion within the Empire contributed to its increasing influence.
The Hohenzollerns instead kept their possessions distinct to maximize representation both in the Reichstag and in the Westphalian and Upper and Lower Saxony Kreis Associations.
Formal political influence was important, because the Hohenzollerns could not compete with Habsburg informal patronage. Although large, the Hohenzollern army and court lacked the prestige and the number of well-paid appointments the Habsburgs could offer, while the emperor had much wider powers of ennoblement.
- Peter H. Wilson's Holy Roman Empire, p. 474Representation in the Reichstag provided a platform to rally support to block uncongenial Habsburg measures, as well as to legitimate Hohenzollerns policy on a wider stage.
Their territories outside the Empire (Hungary and Ducal Prussia) helped, but as extensions of their German realms.