Alsace-Lorraine has been in France for centuries. The area was critical against Germany in both world wars. The territory was briefly part of Germany from 1871-1918 in between wars. Political boundaries should remain the same unless the inhabitants of the region decide differently, especially in peacetime. With a united European Union, the region itself can simply be its own autonomous state as European countries slowly integrate into a more federal system rather than separate on regional lines as in the past 150 years.
Alsace Lorraine was taken by Germany during the Franco-Prussian war when it was largely French. It was returned to France in 1945 following the war. Although the culture is largely German, and the inhabitants speak both French and German, I don't consider that a good enough reason to return the region to German authority. I find it frivolous to redefine the nationality of an area for no good reason.