The best bet if you still want a halfway recognizable Europe is to somehow get the conservative European powers to establish Lotharingia/Lorraine as a buffer state between the restored Kingdom of France and the German states at the Congress of Vienna. Basically covering Lorraine, Alsace, parts of the Rhineland and Greater Luxemburg would result in a nice, little entity, which could be controlled by Vienna to halt French ambitions in Europe. I mean it wouldn't be the sole state between France and the German states (the United Netherlands and Switzerland would still exist after all) but you'd have the classic Franco-German border covered.
And seeing the weird cultural mix happening in that state, chances are that it could become neutral and not join a united Germany (in case the German Confederation ceases to exist) or re-join France (in case they have a resurgence).