That's very interesting. I was expecting the Prussian Netherlands to be a non-starter. Re-the territorial strip to link the two halves of Prussia balanced by a bigger and stronger Hanover...
Looking at the maps of Germany in 1815 and 1867 it seems to me that the additions to Prussia would be Nassau, Hesse-Kassel and Frankfurt. Prussia would also be given Maastricht for a better join between Belgica and Rheinish Prussia and the Dutch would receive Limburg 50 years earlier as compensation. Greater Hanover would be created by annexing Oldenburg, Mecklenburg and Brunswick. But would the rulers of these states agree to loosing their independence at the Congress of Vienna? Or were they not powerful enough have a choice in the matter?
In 1815 the Elector of Hesse-Kassel was not influential enough to get the coveted royal title, so he kept the meaningless electoral title. But Deposing him completely? Unlikely. The Nassau dukes (soon to be combined in one branch) were related to the Stadholder of the Netherlands and also a less powerful, but respectable house.
The simplest way would have been if Prussia kept Hildesheim it got in 1803 and acquired the southern strip of Brunswick. That is basically the Hellweg, an ancient highway for trade and armies, connecting the Lower Rhine with the Harz and Elbe areas. Brunswick could be compensated somewhere else.
(Of course, Prussia would need Corvey and Paderborn as well, the latter it got also in 1803.)
According to a programme on BBC Radio called In Our Time the main cause for the Austro-Prussian War was that Bismarck wanted to annex Hanover to join the two parts of the Hohenzollern monarchy. Therefore if Prussia wasn't divided into two parts in OTL would Bismarck or whoever was in charge of Prussia ITTL be less determined to annex Hanover?
The BBC is sadly mistaken in this regard.
The acquisition of Hanover was very much an afterthought in 66/67.
The fundamental conflict was a) Austria's formal superiority inside the German Bund, and b) the electoral growth of the Liberals who wanted the national unification and threatened to push aside the conservative Prussian monarchy.
The formal reason for the war was the disposition of the duchies of Schleswig and Holstein. Prussia wanted them for itself, in order to build the Kiel Canal and become able to circumwent the Danish straits.
Austria wanted to either deny them to Prussia (so they supported a local claimant) or to get compensated for this Prussian gain (the Austrian chancellor Mensdorff had suggested the cession of parts of silesia to Austria.)
George V. of Hanover just had made the error of supporting Austria. King Wilhelm I. just wanted to take some territory, but Bismarck was convinced that a weakened and humiliated Hanover would just plot revenge, so completely annexing it (against all tradition) was the safer and simpler course.
Would the Prussians try to Germanize the Flemish and Walloons? AFAIK they attempted to Germanize the Poles within their territories IOTL? In the long term could Flemish evolve from a dialect of Dutch into a dialect of German? However, if the Berlin government bungled Germanization and the "mixed marriages" conflict would the result have been disastrous for Prussia in the short term? In this context the short term is 1815 to 1866 and the long-term is 1867 to 1914.
The linguistic policy is hard to predict. There was less germanization in Posen before the Uprising of 1830. Making German the official language of Government in *Belgica would be a big mistake. Even Dutch was difficult to accept for the aristocrats of Flanders (!). And trying to make a Low Frankish dialect into a High German one would be less successful than Volapük.
But anyway, I can easily see high-handed Prussian governors bungling the "Prusianization" of the new territories, leading to an uprising from Ostend to Cologne around 1830, tacitly supported by the Catholic clergy.
Would Belgica under Hapsburg or Hohenzollern rule be part of the German Confederation and
Zollverein? I think it would have been as it had been part of the Holy Roman Empire for centuries. How would that influence the economic development of Belgica? AKAIK Luxembourg benefited economically from its membership of both. Is it likely that the same would have happened to Belgica? If the Flemings and Walloons could see that there were economic benefits to being Prussian would that offset the discontent caused by Germanisation and the "mixed marriage" question?
Benefits compared to what? Th3y cannot compare notes with OTL, and they will probably feel exploited and under-valued as long as their king and government is not located solely in Brussels.