Having the Netherlands holding parts of the Rhineland would do it. They're Catholic and could join the Belgians.
That would butterfly away Belgium.
Not necessarily, a major Catholic revolt would still be likely in an 1800's POD.
give the Belgians more of Germany/the Netherlands in the peace treaty.
As ridiculous as it might sound, they(at least the French speaking elite) actually had some plans for that IRL in 1918!
So yeah, having the Dutch collaborate with the Germans in WW1 would probably enable Belgium to go through with these plans. They'd annex Limburg and the westernmost part of the Rhineland. How stable this state would be is another question, though.