This is really as close to impossible as non-ASB PODs get. I mean, yes, technically it is conceivable that the German Empire introduces a German nobility at the Reich level, but the negotiations were tense enough without introducing that to upset the applecart. Getting the German kings and princes to accept such a title would require superhuman political skill. The titles in Wilhelm's gift were Prussian, not German, for the good reason that creating a German nobility would have lowered the status of all extant nobles (with their now-provincial titles) and humiliated the other kings (who couldn't bestow these new titles).
Assuming this happens, we still need to define what 'German Prince' means. Fürst is a relatively 'soft' title, nowhere near as clearly defined as, say, king or duke. Bismarck was made a non-sovereign prince (for the good reason he couldn't be made a sovereign one without creating a new member state to the Reich). What would a 'German prince' be? It is not really plausible that it would be a sovereign title since that would make him superior to kings (which a prince isn't), but what does a non-sovereign title of that grandiosity actually mean? It has no precedent and is liable to offend.
Even with the princely title, Bismarck still has no executive authority. It is imprtant to remember that 19th century Germany, despite its loud and consistent proclamations of adhering to legitimist principle, was first and foremost a bureaucratic state. Kings did not have power through their blood or the mystical bonds of loyalty, they had power through the law (and often it was mightily circumscribed). The king of Prussia could close down his parliament on a whim, but he could not issue a cabinet order without the written consent of his chancellor (yes, him. Guess who wrote that law). The apparatus of state obeyed because it was what you did. You followed the rules. Max Weber describes this wonderfully in his analysis of the modern bureaucracy, which is really based largely on Wilhelmine Prussia. A 'German prince' has no rules. There is no law pertaining to that position, and it cannot be written against the opposition of the Reichsrat (the princes wouldn't stand for it, even if Prussia overrules them they're likely to walk out) and Reichstag (not a chance, given the sizeable liberal and later Socialist factions). It would have to be a coup d'etat, backed by the Prussian army. Except that the Prussian army would not back it. They're sworn to uphold order and obey the monarch, not to help some crazy nogoodnik politician overthrow the Hohenzollern dynasty. Much the same goes for the civil service. This is not 1933, when a parallel power structure can be erected on the basis of charisma and intimidation. There is no 'Fronterlebnis' to desensitise people to violence, no expectation of messianic deliverance. People won't stand for it.
And finally, assuming the office of 'German prince' is actually created, the moment Bismarck suggests the vacant emperorship might be elective, the first candidate will be a Wittelsbach, not some nobody from the arse end of Ostelbien. We are used to seeing Bismarck through the lens of his cult, but that is mostly an artifact of the early 1900s. While he was alive, the man was disliked even by many of his admirers. Think Rahm Emanuel, the magnificent bastard stereotype. Very few people trusted him. He's not emperor material (granted, neither was Wilhelm II, but that was an accident of heredity. Picking Bismarck would be walking into the buzzsaw with your eyes open).
You cannot do this without changing so many aspects of the German Empire to the point that it really bears little resemblance to OTL.