Well, I mean, it seems nearly impossible- as far as I know, there is no tradition of royal intermarriages between the European monarchies and the Asian monarchies, and I don't know how you'd go about getting one post-1700 as well. And then a personal union? That seems like it'd be unacceptable to one of the two sides, or both. (Depending on whether the family resides in Seoul or Berlin, perhaps)