A treaty passed in early 1814 or late 1814 could have seen Napoleonic France/post-Napoleonic France keeping more of Netherlands. I once proposed something like
this,
in an earlier thread. (maybe adding Luxemburg with it)
I'm afraid to brining in the
Talleyrand Plan, as hundreds of voices are going to call on "cliché".
Eventually the big problem would be dividing up Flanders and Wallonia, the current political situation made it an AH cliché but such thing wasn't that obvious to happen. I could see the division being largely due to British unwillingness to give France any more coastline on North Sea, which may be ignoring linguistical division lines.
One of the latest XIXth attempts may happen with someone less competent than Bismarck in Prussia leading the show, and agreeing with Napoléon III
politique des pourboires, which is allowing him to annex territories west of the Rhine in exchange of his "benevolent neutrality"
In 1866, there was such attempt with several plans on Belgium, Luxemburg and Palatinate.
If both France and Prussia act out of their minds (and that would backfire), maybe this could have been attempted.
Eventually, the most plausible XXth possibility for me would be a situation with an equivalence to the
Congrès de Liège but voting only once as the IOTL congress did
at first in October 1945.
IOTL it was decided ultimatly in favour of federalism at a very large majority, but maybe an even more divided Belgian public opinion on the king (obviously collaborating ITTL?)