Danish king marries Swedish king's eldest daughter. He (Oskar ii?) Had many daughters but no sons. If Christian (ix?) Is married to her then the Danish royal family unites Denmark, Norway, Sweden, Iceland, Greenland, and might claim part or all of Finland later. They might even have a shot at Estonia and Latvia under the wrong circumstances.
A permutation on that would be having a German Emperor marry the daughter of the Swedish king and in turn having the eldest son of that union marry Wilhelmina of the Netherlands while permitting Denmark into the North German Confederation in 1865 - something the Danish king actually proposed but Bismarck turned down. Again, under the wrong curcumstances, it may be possible to have a super-Germany running from Antwerp or Amsterdam to Tallinn with every piece of Baltic acreage save St Petersburg in German hands. Berlin would rule Northern Europe, especially if Congress Poland were integrated directly into this uber-GröBdeutscheland