So, are we looking at a confederation of German States? Under the guise of the HRE?
I think the best way would be to united many of the princes, with roughly two in each major region, but crucially, prevent the rise of any single dominant player (say a no-inheritance rule, son cannot stand just after the father?) and then create a genuine Imperial Territory that is essentially ruled by Bureaucrats that serve the Empire rather than any single Empire.
Two rather large changes - but with fewer, stronger princes, no single state can easily dominate, and no inheritance means no situations where the Imperial Territory is constantly used to fund the same dynastic territories development.
The only issue is going to be border states, if they can expand outside of the Empire, without the Emperor reigning them in/confiscating territory, then they have no way to control the expansion. Perhaps if external wars are made illegal without Imperial involvement/decree?
i.e. Yes Brandenburg, you may conquer Polish territory, but you will give the territory to your 3rd son to rule afterwards, have 10,000 men.
TL;DR - you need to limit the number, and strength of personalities involved, and ensure that no single state can ever dominate - the problem with this is fundamentally that the HRE allowed internal warfare - and without that, the HRE stops being the HRE post-Golden-Bulle, and starts become a giant german feudal state.