The Belgian monarchy may come under some pretty hair-raising struggles as the country may split in twain, but it held together after WW2 and Leopald's Nazi collaboration so im sure they'll at least rule a rump Flanders.
Color me skeptical. It seems unlikely to me that the monarchy will continue to reign over Flanders if Belgium is to be dissolved. While Flanders saved the monarchy in the post-war referendum (Walloons voting against), the monarchy has lost much of its initial popularity in Flanders: it is, of course, seen as a very "Belgian" institution and, as such, diametrically opposed to the goals of the Flemish nationalist movement, sometimes even explicitly. Some of the members of the royal family don't even seem to be able to speak in Dutch properly (and among our lovely southern neighbors the concept of "properly" is quite stretched already
). In sum: if Flanders is to be independent, I don't think it will be a monarchy (unless it decides to become part of the Netherlands, which seems extremely unlikely - in which case it gets another monarchy, the Dutch one).