Very unlikely.
In 1866, Prussia defeated many other German states and annexed three of them. But some fought on Prussia side: the Mecklenburgs, Oldenburg, Anhalt, Brunswick, Lippe, Waldeck, some Thuringian statelets.
If Prussia formally annexes them afterwards, that would be a huge diplomatic blunder. And practically a guarantee that the Southern German states will not ally themselves with Prussia in 1867 and fight for their independence at the side of France.
OTOH, Prussia annexing all of the defeated enemies in 1866, including the GD of Baden and the Kdms of Württemberg, Bavaria and Saxony, that will overreach what France (and certainly Russia) are willing to concede. There will be the Prussian War of 1866 to ~68, and it will end with a humiliating defeat for Prussia, Austrian Silesia, Russian Posen and a French-led Second Rheinbund.
And after 1871? Wilhelm I was so honor-obsessed, he would never stand for betrying his princely allies this way. The first Hohenzollern I can see who would be willing to go with practicality and ignore traditions when it suited him would have been Crown Prince Wilhelm, the hypothetical Wilhelm III.