A Napoleonic Wars POD does seems like the best bet if you still want Prussia to really be cemented as a thing, without being part of Germany.
Following the current discussion, a strong Scandinavian power* might be useful. It could act as a counterweight to the Prussians, reducing their pull in northern Germany. Combine this with the Prussians being confined to the territories in the east when the wars are over, and you might just be left with a Prussia with enough power to not just roll over, but too little to upset the status quo on its own.
Going forward a few decades, an alternate 1848 might see most of Germany united, while the Prussians fall increasingly under the influence of Russia. I don't see the Russians taking kindly to Prussia being absorbed into a "liberal" German state, so propping up the Hohenzollerns would make sense. If Austria is part of this Germany, I suppose we could then assume that the Hungarians are running the show in the remains of the empire. All three have an interest in keeping their ethnic minorities and peasants down, so they could form a conservative/reactionary block against the liberals of Germany and Italy.
*Danish king is a bit smarter and joins Britain against Napoleon, and is rewarded with Sweden (without Finland of course) at the end of the Napoleonic wars? Just not losing Norway might be enough though.
It's a pretty interesting question. It's not just a difference of some territory, it might be the character of the entire state. Much more Catholic, more liberal (assuming a revolutionary birth) and more focused on the south. And Prussia, sitting right on a major invasion route between this liberal Germany and the reactionary Russia would be in for some interesting times. I might cook up some simply maps of some scenarios tomorrow.
Also, just thinking about it and from Iori's map, a Germany that included Austria but not Prussia would be one hell of a awkward looking Germany. Maybe thats coming from my Victoria based desire for pretty borders, but its not great looking, you gotta admit.
Now imagine that it somehow included Denmark as well!
