Whatever its inhabitants wanted.
Except that popular self-determination was a concept still making its way into geopolitical consciousness at that time.
France lost Alsace-Lorraine because she was defeated in war, crushingly - plain and simple. Without the war, the territory is still French in 1872.
And yet, this does not end the analysis. Alsace-Lorraine was French in 1870 because French arms conquered it, French arms retained it, and no foreign power had been inclined to challenge it for some years. Louis XIV did not poll the inhabitants of Alsace when he annexed it, and Louis XV did not do so with Lorraine, either. They were taken by force, and what the locals wanted was neither here nor there.
By the early 20th century, however, such concerns could no longer be ignored. Of course, self-determination can be messy, too, especially when it turns out that you can't produce clear borders with it, and mutually antagonistic groups are living cheek by jowl with each other (see: the pre-war Austro-Hungarian Empire).
Had Germany not gone to war with France - or had won it - I expect this would be an unlikely exercise even on alt-history forums; Alsace-Lorraine would be so much a part of German society that it would hardly be questioned outside the most revanchist fringes of whatever existed in French society. Even with the Zabern Affair, it was still largely reconciled to German rule by the outbreak of the Great War, and local autonomy parties could no longer poll a majority in elections.
Again, though, the smart play would have been Bismarck's: given them ample autonomy and self-government from the start. The result could well have been communities with such a strong sense of self-identity (an identity more in tune with similar communities on the other side of the Rhine than Paris, to boot) that any French reacquisition would have been a very difficult feat, and the more likely long-term result would be one or more independent states. French demands for its return would ring increasingly unrealistic and quixotic on the larger world stage that was ever more sensitive to self-determination.