WI Danish-German alliance a century earlier?
The prospects mentioned are enough to make me wonder if a union of some degree (personal union leading to permanent federation and eventual national merger perhaps?) could have been possible between Denmark and neighboring German powers a century earlier still. Hannover comes to mind. Vice versa could Denmark as such have aggrandized on the Continent to the point that a majority of the population under the Danish crown was in fact northwest German?
This might put a check on German unification, with the Danish holdings (assuming here they hold firm, having accepted the rule of the Danish dynasty and not feeling a disability for speaking a different dialect than the ruling core of the total kingdom, and the kingdom can fight off German and other neighboring rivals well enough to keep the territory more or less intact) being as indigestible to a unifier such as Prussia or Saxony (or Hannover, assuming that kingdom is not in the Danish holdings) as Austria was to Prussian hegemony OTL--so there would be Germany, with or without Austria, then a greater Denmark to the north.
But could the Danish dynasty, having proven themselves to be acceptable overlords to their German subjects, be the one that unifies Germany?
Linguistically speaking, of course the range of dialects we call "German" branched off the Scandinavian ones a very long time ago. But the north German local dialects differ from "High German" by quite a lot, being closer to the lowlands languages (Dutch, Flemish) which are in turn remarkably close to English, or anyway what English would be like without the Norman Conquest. Denmark, as a kingdom, has an ancient history by European standards, going back a thousand years even in the 18th century. So I wouldn't foresee an actual merger of Danish and German identity, but is it unthinkable that Germans would come to see membership in the expanded Danish empire as part of their identity?
Yet another possibility would be if an alliance, perhaps secured via intermarriage and thus intertwining the dynasties, would tie Denmark to a northwest German principality that then goes on, using the assets of Denmark as leverage, to unify the northwest and from there become the nucleus of German unity. Here the unified Germany under a northwestern Kaiser is not under the Danish crown; rather Denmark is distinct but strongly tied to the German empire, much as Scotland and England were intertwined before the Act of Union. And perhaps there would even be an Act of that kind and Denmark would relate to Germany as Scotland does to England--both with distinct institutions and of course languages, but under one monarch and one central legislature (if the sorts of princes who set their eyes on unifying Germany would even countenance a legislature.
Bonus points if the German house is Hanoverian, or some other one but the British choose their post-Orange dynasty to import from there too, so we have in the 18th century Britain, Denmark and holdings, and a good chunk of North Sea and/or Baltic Germany all under the same monarch, and if the continental powers will harmonize their succession laws with Britain, eventually a Germanic-language superpower from Prussia to Connaught, from Svalbard down to Bavaria, with the Netherlands and Flanders conceivably getting pulled in too by the sheer mass of it!
I have to admit that that's a bit too dizzying to contemplate, but if we recall that in the 18th century "Denmark" included not just the overseas territories Sian listed but Norway as well, then a fusion of that Denmark and Germany would be a grandiose empire indeed, even if partitioned into two subempires.
Could the Danish Navy, subsidized by the German portion of the realm (as well as the ongoing Oresund revenues--if Dano-Germany survives as a power comparable to growing Prussian hegemony of the 19th century, no one is going to be in a position to demand the Oresund tolls be abolished until the whole meta-kingdom is soundly defeated) make itself a match of the RN, or even eclipse it?