During the First World War, the German leadership planned—in the event of a German victory—to annex portions of eastern Belgium, and to transform the remaining territory (perhaps with the addition of some annexed from France) into a vassal. This new state was to be known as the Kingdom of Flanders-Wallonia, which raises the question: who was to be its king? Would the Belgian king be left in place, would a new monarch (such as a former German prince) take his place, or would the kingdom have been placed in a personal union with the Kaiser himself—as was intended for the United Baltic Duchy?
who was to be its king? Would the Belgian king be left in place, would a new monarch (such as a former German prince) take his place, or would the kingdom have been placed in a personal union with the Kaiser himself—as was intended for the United Baltic Duchy?
Most likely a prince of some kind from one of Germany's kingdoms. For example, one of he Kaiser's sons, Joachim, was being name dropped for King of Ireland, in a no doubt extremely implausible scenario for the Easter Rising in 1916. During the war there was something of a feud going on between many of the royal families of Germany over territories that they might rule and inherit, so there would certainly have been no shortage of volunteers for the post of Flemish-Wallonian Monarch!
If you want a good run down of discussions on German War Aims, German War Aims in the First World War by Fritz Fischer remains the best volume out there.