What if Belgium and Limburg became part of the German Confederation like Luxemburg after the Treaty of Vienna, but still was part of Netherland?
How would this change history?
http://en.wikipedia.org/wiki/United_Kingdom_of_the_Netherlands
Well, then logically an independnat Belgium would most likely be part of the German Cofnederation, too. Barring butterflies preventing that war, that would mean that most likely belgium would take a side in the German War of 1866. Or any equivalent war, of course.
If they take the Prussian side and win, it may become part of the North German Cofnederation - and hence of the German Empire later on. If they dont become part of the NGC, they will most likely stay within the German Customs Association like Luxemburg did IOTL, not a great change but something. If they take the Prussian side and lose, then nothing, GC is restored and thats it, though a butterflied Austrian victory would of course drastically change the political future.
If they take the Austrian side and win, then again nothing, see above. If they tke the Austrian side and lose, thing sget intersting: Prussia might try to annex it, too, but there raethe french to consider, who are itching to intervene in some form, no matter how (hence Bismarcks fast peace with Austria, IOTL). They could take that as a pretext to intervene against Prussia, or maybe Prussia is quick enough there, too...
Before any inner-German war it would have little impact, though, IMO.