Successful
Louis XIV campaigns and annexation over the Rhine could be really helpful.
Basically, we could end with French Flanders, Luxemburg and/or Palatinate which were only temporary annexed IOTL.
As for Burgundy being culturally French, I think it's clearly most complex : Burgundian chancery and court certainly were as most of their holdings; but others (such as in Flanders) were since a long time autonomous including culturally and Burgundian court never really managed to get a cultural grasp on that, quite at the contrary).
Last realistic possibility is, IMO, French Revolutionary Wars in a TL where Montagne is still in power : they didn't believed much in an all-annexation geostrategy (as Directoire did), but rather to set up other pro-french republics such as the
Republic of Mainz.
With enough Montagnard France's success, it could realistically be considered culturally as Franco-German.
Anything past that is, because of France's relative decline in the first part of XIXth century and rise of German nationalism, hugely unlikely. There's a reason if annexation or sattelisation of Rhineland was never really considered by France in WW1 (in spite of tentatives to make it appear as real goals of war (generally to make them equivalent of German plans of annexation), it was essentially far-right wet dream and nothing more).
The only think I could see as doable after the XVIIIth would be durable sattelisation of Saar/Sarre, and even that would require major changes : and forget possibilities of annexation trough referundum. That's simply not going to happen, while blunt annexation in a worse peace for Germany is technically possible (but you can bet that it wouldn't acculturate easily safe important population change, which would be hard to do, would it be only politically)