Let's say the Luxembourg Crisis results in war.
In 1867 the Prussian Military Reforms that were formulated from the Prussian experience in 1866 were not yet implemented. Prussia will also lack as developed a rail network as it did in 1870. Thus, Prussia will not have the mobilization and transportation efficiency that it had in 1870 OTL which allowed Prussia to outnumber French forces 2-1 early on. For good measure,
France ends up victorious and annexing Luxembourg, the Left Bank of the Rhine, and Kehl. Short of marching on Berlin, I don't think France could undo the establishment of the North German Federation - however the treaty put in place forbids the South German states from forming any broader entity with the NGF. For good measure, Schleswig is returned to Denmark (let's say the Danes sided with France in the war).
Most of the German Empire's industrial cores (the Westphalian Ruhr, Elbe-Saxony region, and Silesia) are still in the NGF here, so despite having less manpower the NGF should still be a force to be reckoned with. France, however, has the Gibraltar of the North (Luxembourg), the Saarland, a small bit of the Ruhr, and a larger population (although most likely something of a restive one, though the importation of Poles to the Ruhr which happened OTL might change things). Plus France has Alsace-Lorraine still.
Belgium will likely be very antsy, as will Britain. The Italians don't necessarily hate the French yet (the big disapproval-spike happened when they shot at Garibaldi in November of 1867 - if Napoleon leaves Rome unguarded for this war as he brings the garrison to France that problem more or less solves itself). Empress Eugénie and Queen Isabella wanted Napoleon IV to marry María del Pilar OTL, and the two were apparently quite in love historically (so much so that Maria may have killed herself when Napoleon IV died). I could see Spain becoming an ally/junior partner to the French Empire.
Other thoughts?