The French saw the Prussian victory in Königgrätz as a blow to their interests, thus Revanche pour Sadowa.
But having the Austrians win, even win big, is far from impossible. Vienna has already agreed not to seek total dominance of Germany. At most, they will ask for Silesia, recreate pre-1815 Saxony and give parts of the Prussian Rhineland to France and to the Archdukes of Tuscany and Modena. Maybe Hanover can translate its victory at Langensalza into the acquisition of some small bits, like Minden and perhaps Lauenburg. Schleswig-Holstein will likely get an Augustenburg duke.
Russia will diplomatically speak against a further dismemberment of Prussia.
Having gained the Saar/Trier area, and possibly the option to buy Luxemburg, France will have made a visible gain from intervening in German affairs. I doubt that they will seek another war there in the next few years.
OTOH, Napoleon III might see this outcome as an invitation to further meddle in the affairs of the German states. Who knows, he might even see it as his historic mission to create a second Rheinbund by pushing both Austria and Prussia out of the Third Germany.