Even though I love the idea, it will be hard. Your POD is before the idea of European cultural superiority, so that is not a problem (although it would have been in the 19th century). But a bigger problem is religeous. In the 17th century protestants didn't even marry catholics (and vice versa), let alone heathens (ok, in the eyes of the protestants catholics were as bad, or even worse as heathens). So to solve this problem you need to make Japan, or at least the royal family, christian. Another solution might be to delay any royal marriages until later, probably not earlier than the 19th century, when marrying non-christians wouldn't be looked on as bad, but you need a very big change in how people think. I guess only now in the twentyfirst century people won't mind their royal family marrying a non-christian (much). I suggest some other royal marriages a bit earlier to non-christians, first with Muslims or Jews (whose religion is a lot closer to christianity) and later to other religions. For example France marrying into the Ottoman empire, or the Brittish into one of the Mughal families or something like that.
Another problem is the Dutch republic, but that can be solved. If you use a early POD the Netherlands can easily become a monarchy, something every stadholder tried BTW, so you could even have one of them succeed in it.
Another important problem is the distance. The Netherlands-Japan is a long way. You can't have a king/emperor rule both countries at the same time, they are just to far from each other.
These are the problems I see from the Dutch spoint of view; there are no doubt lots of problems from the Japanese Point of view I am not aware of. A Japanese-Dutch personal union is hard, if not impossible. I think the easiest way is a future timeline in which Amalia the oldest of the Dutch princesses falls in love with the male heir to the throne in Japan and marry (although I believe that male heir must yet be born), or possibly even farther into the future with the children of Amalia (who is only 6 btw).
Another problem is the Dutch republic, but that can be solved. If you use a early POD the Netherlands can easily become a monarchy, something every stadholder tried BTW, so you could even have one of them succeed in it.
Another important problem is the distance. The Netherlands-Japan is a long way. You can't have a king/emperor rule both countries at the same time, they are just to far from each other.
These are the problems I see from the Dutch spoint of view; there are no doubt lots of problems from the Japanese Point of view I am not aware of. A Japanese-Dutch personal union is hard, if not impossible. I think the easiest way is a future timeline in which Amalia the oldest of the Dutch princesses falls in love with the male heir to the throne in Japan and marry (although I believe that male heir must yet be born), or possibly even farther into the future with the children of Amalia (who is only 6 btw).