About German parts becoming part of the Netherlands:
With an 17th century or even an 18th century POD I think you can effectively double the Netherlands in size and the Netherlands can still completely absorb and assimilate it into the Netherlands. The Dutch republic probably was that influential in those days, while German nationalism was still too unimportant.
That said, after the Napoleonic wars everything changed. The Dutch economy and influence had already been in decline during the late 18th century, but the French domination and later occupation of the Netherlands completely ruined. The Netherlands emerged out of the Napoleonic wars a poor and broken nation. It economy was in ruin and its reputation completely. Respect for the country was at an all time low. At the same time German nationalism was on the rise. Prussia and Austria were two of the main countries that managed to defeat France. While the Netherlands was on its way down, Germany was on its way up. People in the neighbouring lands started looking more towards Germany than towards the Netherlands (a process that had already started in the late 18th century BTW). Simply put, it would have been a lot harder for the netherlands to absorb parts of Germany than it used to.
Mind you, I believe it still could. At least some minor parts that already had a strong connection to the Netherlands. For example it managed to asimilate northern Limburg, which had been Prussian for a while. The same is true for some former minor enclaves. So I believe that the Netherlands could have absobed more of Germany, like East Frisia or Cleves or Eastern Gueldres. It could not have absorbed large parts, like the Rhineland. A Dutch Rhineland means trouble with the rising Germany in the long term. I realy doubt the people of Koblenz wanted to be Dutch and I don't think Willem I could have convinced them. So I believe expansion east (except in smaller, less important areas) is out of the question for the Netherlands with a POD around 1814.
As I said, that basicly leaves the Southern Netherlands. A more competent king or crown prince could have kept the Netherlands together. With the Wallon industry and the Dutch trade empire the Netherlands will be a force to be reconed with. Surely not a rival of Britain, France, Prussia, Austria or Russia. But above Spain and Italy? Certainly possible. Especialy if they are able to do well on the colonial game. Mind you Indonesia already was the big price. Probably the best colony around (I would say only India is its rival).