Or maybe some of the western states would prefer to become a part of the Netherlands?
I doubt it? There was a Dutch identity based on longstanding territorial borders and some cultural attributes. I can see places right on the border of the Netherlands, like East Frisia, joining in, but Bremen and Hamburg wouldn't, and from Cologne south, the Rhineland itself speaks High German.
For what it's worth, in my TL there's an unmentioned pan-Lotharingian identity in some of the liberal or national-liberal provinces near the Franco-German border: Alsace, the Rhineland, Frankfurt, Holland, Brabant, and to some extent Baden and the Palatinate. But there are no real demands for political secession (Frankfurt is the capital, and Alsace and the Netherlands got their federalism and language rights long ago), just international cooperation, same way there's cooperation between OTL's New York and Ontario on Niagara Falls infrastructure.
The issue here is that, if there's a stable liberal government rather than Austrian domination, there is no need for the Rhineland to be resentful. It's going to be rich and a net tax donor, yes. But OTL's New York is also a rich net tax donor, and yet pretty much nobody proposes secession (and if they do, it's of the city and not the state). Secession is the domain of areas that view themselves as distinct from the entire country in identity: Quebec, Catalonia, Scotland, Northern Italy, to some extent Bavaria. Regions that perceive themselves as a dominant or formerly dominant elite don't do that, which is why the Northeastern US never talks about secession and California only does in jokes.
If *Germany got over the hump of *Austrian domination in Grossdeutschland unification, that's how the *Rhineland would view itself: like New York or California, and not like Catalonia. The most that could happen is a unified Rhinelander identity demanding a state within Germany, the way some interwar German politicians, including Adenauer, wanted (North Rhine-)Westphalia to secede from Prussia and be a separate state within Germany.