Assuming that full independence was not acceptable nor was staying under the Habsburgs, they had to join someone.
What would be the will of the people?
Flanders - Dutch speaking Catholics - Would they prefer their co-linguists in the Netherlands or co-religionists in France?
Walloonia - French dialect - Catholics - I don't see anything but France in this one.
Luxembourg - Lexembourgish speaking Catholics - The Wittelsbach Palatinate territories of Berg and Julich?