Problem with southeastern Belgium and the Rhineland is that it put's the Netherlands in between France and the Germans (in whatever configuration). At the Vienna Congress that might have been seen as an advantage to the english and perhaps the Prussians. As a Dutchman however I would prefer to be left out of that as much as possible.
The most likely border, culturally, economically, militarily and certainly when one considers the language, would be The river "Maas". The cities Luik (Liege), Namen (Namur), Karelskroon (Charleroi) could form the border with a small and mostly neutral Walloon-Luxemburg. A Country that would not form a threat to any party, and would prevent the situation that the Dutch get involved in every Franco-German conflict. The latter could exhaust themselves doing so, while the Dutch would be able to more rigorously consolidate their trade power and their world empire then in OTL.
Duinkerken (Dunkirk) would be an excellent south-western outpost of the United Netherlands, with Rijsel (Lille) and Bergen (Mons) as the garrison cities connecting with the previously mentioned Maas-Line Cities. Off course all the French names of these cities would vanish in TTL.
The United Netherlands without southeastern belgium and the Rhineland, would still have twice the mass compared to the Netherlands in OTL. Large enough to stand our ground worlwide for another 200 years; but yet small enough not to provoke the superpowers too much.
Could anyone help me find a good website to manipulate maps. It would help very much to visualise something once in a while.