I also think that a plebiscite in Alsace-Lorraine without the expulsion of immigrants since 1871 that goes out in favour of Germany is the best possibility, together with France needlessly and irrationally alienating all other Allies.
Problem is that this would likely require a much earlier POD. If Alsace were made a full member state of the Empire right from the beginning or at least very early, then at the end of WWI it would have a history of almost 50 years of internal autonomy that it would never enjoy within the French system. Furthermore, it would have been more unikely that the Prussian soldiery bosses around Alsaciennes the way they did IOTL when those are not second-class citizens and have an own state to intervene. With those changes I could see a majority in Alsace voting for staying within Germany. Federalism definitely has advantages for minorities with a distinct identity that a centralist state like France cannot offer.