"Give the Belgians Artois, perhaps, in exchange for the Belgians acquiescing to a German annexation of Luxembourg?"
I can't imagine Belgium wanting any French territory, it would have been a danger to her survival. From France wanting the territories returned, also from the resentfulness of the French speakers forced to join Belgium.
Yes Belgium already has a lot of French speakers, to add more might lead them to wish to join France. Might upset the Flemish as well, they might demand to join Holland.
The subject was AL not been annexed, i think it's Germanys best option as it leaves France without anything to fight another war for. I think no annexions, maybe a pleibecite in Alsace brings a situation where the pot can cool down. Ok it might takes decades, but ends Frances territorial demands in the Rhine area.
If i had been the Kaiser i would have tied non annexaton and peace for France - Alsace Pleibecite - to a treaty that recognised the permanence of the Franco German, Franco Belgian, and Belgian German borders.
Know this would have won favour with Britain, and possibly offered more security.