The monarch will become a figurehead in due time. But as the country becomes democratic, how will the Walloons tolerate a system in which they will be outvoted like 6 to 1 by the Dutch speakers?
I know right now in Belgium, Wallonia has a ton of independence; according to Wikipedia "The Walloon Region's autonomy extends even to foreign policy; Wallonia is entitled to pursue its own foreign policy, including the signing of treaties, and in many domains even the Belgian federal government is not able to sign an international treaty without the agreement of the Parliament of Wallonia". So maybe generations of royal govenors will be able to negotiate a similar position for them in the Netherlands.
Wallonia was and is a huge moneymaking area: "In the 18th century, Wallonia was the second industrial power in the world, in proportion to its population and its territory, after the United Kingdom... currently Wallonia has some companies which are world leaders in their specialized fields, including armaments, glass production, lime and limestone production, cyclotrons, and aviation parts." From what I understand, this is part of the reason they have the independence they do. I think the monarchy could be persuaded to forgo a little control for a better overall economy / richer country.