Accepting that new areas are incorperated more rapidly would probably be the best way to do it ... it could certainly grab Denmark (via the danish king's ownership of the Duchies of Holstein and Schleswig) and depending on your specifications (do it need to be germanic in culture?) also Habsburg lands
Are you talking about the 1864 suggestion by the Danish king to join the German Confederation here, or an earlier POD? Because in 1864 that plan would be shot down so fast by anyone but the king. Neither the French, the Russians nor the British will stand for it, and neither will the Danish people. Not to mention Bismarck thought it was a stupid idea as well.
An earlier POD, such as the Danish king being a bit braver and actually managing to gain a strong position in the north during the Thirty Year War could possibly, maybe, see a German state that includes Denmark. Considering how much influence German had, and how much more it would gain as Denmark expanded into Northern Germany, I don't expect the Germans to be particularly upset about this. Especially not when Copenhagen becomes more German than Danish, as Germans flock to the capital.
It would certainly be a different Germany if it's evolved out of such a state. The Danish naval focus, and the trading tradition of the North German cities mesh pretty well, which could mean colonial expansion beyond what Denmark managed in OTL. Perhaps the wealth from that could be leveraged in the Eastern Baltic, (re)conquering areas that had been a focus for both parties in the past. Those areas were not exactly densely populated, so a Germaniziation could happen. There's certainly the ingredients for a massive population boom if the Ruhr is gobbled up at some point, which has the most immediate outlet in the east.
Of course, you still need to actually unify Germany, but you'll at least have expanded into both Denmark and the Eastern Baltic. Oh yeah, Norway would come along in this deal, as would Iceland and Greenland. The former two seem likely to break away at some point though.