Ok, I was thinking about it and was wondering how plausible this scenario is:
POD around 1650. Stadholder Willem II doesn't die and manages after a small civil war to break the power of the states of Holland (the province of Holland, not the country of the Netherlands) and so increases the influence of the stadholder. (OTL Willem II did try to break the power of Holland, but he died before he was able to). He fathers two children, a son Willem (OTL William III of England) and a daughter (let's call her Anna). Willem marries into the English royal family like OTL and Anna marries the ruler of Hanover (wasn't it called some kind of Brunswick at this point).
Because the original poster wanted Belgium as part of the Netherlands so I suggest this. Before Willem II died OTL he tried to make a deal with France to divide the southern Netherlands. Now he continues this plans and makes a deal with France and they go to war with Spain. End result: the Netherlands adds a large part of Flanders, Brabant and Gueldres to its territory, while France gains the rest. I suggest the line Ostend, Ghent Mechelen (I think the Netherlands gains all Dutch speaking parts and France the rest is a bit cliche). The Netherlands also gain some Spanish colonies.
Sadly Willem II dies during this war. The merchant-regents in Holland decide to get rid of the stadholder (as Willem is still too young). This ruins the Dutch French relation (which would have been ruined anyway as France wanted the rest of the southern Netherlands for itself and part of the Netherlands too). Anyway a bit later France decides to attack the Netherlands, together with England (or maybe not England, it doesn't realy matter) and Munster/Cologne. As OTL this goes badly for the Netherlands and a large part is overrun by France. The Dutch beat the English at sea, the desperate population blames the regents and Willem becomes the new stadholder. He manages to beat the French by gaining some powerful allies, which include Hanover (or Brunswick-whatever). After the war the Dutch want the French as far away as possible so the border shifts south, so the Netherlands now include Dunkirk and Brussels (still not all of Belgium I must admit) and it is heavily fortified. To prevent an attack from the east (and as a reward) Hanover annexes a large part of Munster.
Willem III still dies childless. His nephew (the son of his daughter and now (or soon to be) ruler of Hanover is apointed stadholder and inherits the various personal titles, like prince of Orange and count of Lingen (which is conveniently close to recently annexed Munster). We now have the ruler of Hanover stadholder of the more important parts of the Netherlands and ruler of important territories between Hanover and the Netherlands. The biggest problem is how do we let the states of Holland/Utrecht/Zeeland/Overijssel and Gueldres accept him as stadholder. Maybe only Gueldres accept him as stadholder, making the other parts stadholder free until the French start acting up (which is probably very quickly).
Ok, how likely is this?