This control of Frisia would certainly provoke Otto von Bismarck to try and defeat the Netherlands to gain a German territory. IF you wish for a way to get the Netherlands into World War I, make them lose that war and there you go.
I am planning a real different German unification (well not really a unification). My idea was that during the 40's revolts Prussia conquers a lot of those tiny German states, who will try to form republics. In the end Prussia controls most of Germany, with the exception of Bavaria and Austria. World War I (or that timelines variant of it) will be Prussia trying to add Bavaria to its territory. France, who is Bavaria's ally declares war on Prussia and the war is started. Prussia decides to attack France through the Netherlands, so he can also add Luxembourg and East-Frisia.