Maybe if Germany had NOT taken Alsace Lorraine in 1871 the relations between F+G would have been "better" in the years after 1871
In retrospect, yes, but Bismarck had shot his influence stopping the Prussians from taking bits of Austria (mainly the Sudetenland, IIRC) and, thereby, making them the enemy (making Prussia face the prospect of a two-front war against France as Austria joins in on the fun). Thus, he was basically forced to take something from France and A-L was the only thing on offer.
Now, granted, looking back, picking good relations with France over good relations with Austria would probably be the better idea, albeit one arguably worse for Germany as France's relative power means it's less likely to be reduced to a German client and would be more of an equal and an ally.
As for the OP, the obvious choice would be the Austrian victory in the Austro-Prussian war, since Prussia would be in no position to try anything after it. Otherwise ... well, it's not exactly unavoidable, but certainly necessary for German unification to go through. France cannot allow a unified German state to form on it's flank, so they'd have to fight a war unless Prussia decides to give up on it's ambitions regarding the unification of Germany (not something likely to work, they were committed to the path).