There are many ways to avoid Belgium, certainly with a POD far enough.
First if all the principalities that made up the Netherlands and Belgium aren't unified, both Belgium and the Netherlands will never exist and end up French, German, or something completely different.
Next avoid the Dutch revolt somehow. If the Netherlands didn't end up Spanish, but Austrian if Philip II had been a better king, etc.
Next let the entire Dutch revolution fail. All of the Netherlands remain Spanish, no Belgium.
Next The Dutch revolt is more succesful and manages to capture what is now Belgium.
Next the French are more succesful in the 17th and 18th century and capture the Spanish or Austrian Netherlands.
Next the Dutch care more about the southern Netherlands in the 17th and 18th century and actually succesfully try to add it to the republic. (or possibly a combination of the last two with the Dutch and French dividing it somehow).
Next The French manage to keep it after they conquered it during the French revolution.
Next the Prussians keep the Southern Netherlands, which they occupied after the Napoleonic wars.
Next the Belgian revolution is avoided, failed or is defeated by the Dutch.
In the peace settlement after the Belgian revolution Belgium is divided between France, the Netherlands and maybe Prussia (possibly including a independent state around Antwerp). The Talleyrandrand or Talleyrand-Wellington partition plans.
So oppertunities enough to avoid Belgium.