Wouldn't this Germany also have to include Bohemia if it's going wider a field? Sure, the national revival was going on, but if we're going with a Grossdeutsches identity/popular movement, you have to figure Czechs are probably going to get swept in - they're in the German Confederation already, after all.
Lots of outcomes are possible. We have Prussia, which holds lands in the east (a considerable portion of which majority-German) that are
not in the Confederation. Then we have A-H with its own problems: a big and distintly
non-German part in the east. There were all sorts of proposals at the time, incuding an Austrian one to turn the whole shebang (
including Hungary and Eastern Prussia) into a vast multi-ethnic (but German-dominated) empire under a Habsburg Emperor. Naturally, the Prussians weren't inclined to go for that. The Hungarians weren't keen, either-- to say the
least. If Pan-German(ic) nationalism is the premise, then there might be focus on separating German and non-German possessions. That is: everything German(ic) would be desired as part of Greater Germany, while all the non-German(ic) bits would be separate entities existing in personal union. Much like
@TruthfulPanda, I see this as a greater mess and obstacle than Prussia gaining Denmark. On the other hand, pure fervour can get things done. It
has, throughout history.
As far as the actual subject of this thread is concerned, I can imagine two basic scenarios (derived from the same nationalism-related premise) that would make it work:
1. Pan-German(ic) fervour is raised, leading to much broader support for a Greater German solution, and popularising the idea that all Germanic peoples should join together somehow. This leads, over Prussian objections, to a very loose Greater Germany under a Habsburg Emperor. To increase Prussia's share, the eastern parts of Prussia are included, whereas the Czechs in Bohemia are not. (Would the Sudeten areas be split off into a separate province of Austria, as was from time to time suggested? Perhaps, because the ethnikc element of the nationalist ideal is rather prominent in this scenario.) Likewise, the eastern half of Austria-Hungary is also excluded. The Emperor of Greater Germany is simply
also the King of Greater Hungary and of Bohemia. This still makes Greater Germany way too Catholic for Prussia's liking. Therefore, tapping into the pan-Germanic sentiment of this ATL, Prussia
definitely takes its chance to incorporate Denmark and add more Protestants. This scenario would definitely make all neighbouring powers wary of Greater Germany. Wary in the
extreme. There will not be a Franco-Prussian War for obvious reasons, but there may well be an earlier Great War (a.k.a. "Germany-and-countries-in-personal-union-with-it v. Everyone Else"). It also demands a lot of fussing, negotiating, re-organising, settling, compromising, etc. etc. -- which is why I think it's the less likely scenario. It does, however, give us a Prussian-owned Denmark, with plausible reasons for Prussia to want this.
2. Pan-German(ic) fervour is raised, leading to much broader support for a Greater German solution, and popularising the idea that all Germanic peoples should join together somehow. This
almost leads to a very loose Greater Germany under a Habsburg Emperor-- but Prussia manages to stave this off for the time being. The idea of pan-Germanism can't easily be killed off, though. Prussia is very worried about being shuffled into an Austrian-led Germany. Religious differences are deliberately played up. When the time comes, Prussia eagerly takes control of Denmark, to gain more Protestants and to orient itself northwards-- away from Austria. Prussia enters into a further treaty of alliance with Sweden-Norway to further this end. Maybe a neo-Hansa is set up? This all has the effect of pushing Bavaria, Baden and Württemberg towards Austria. Eventually, we see an overwhelmingly Protestant North Germany and an overwhelmingly Catholic South Germany emerge. These can be countries or tight alliances, that hardly matters for the scenario. They're
not friends. There's no need for a Franco-Prussian War. With Germany divided, a Great War (at least the OTL one) also looks less likely. I consider this both the more realistic and the more interesting scenario for getting Denmark owned by Prussia. (What happens to Austria and its many non-German possessions cannot be accurately predicted without devisig a more detailed sequence of events.)