This is far-fetched and so, but still interesting discussion even if there are zero chances of such a thing ever happening.
Anyway, the German Empire when formed in 1871 was a federal state, much like the US, Canada and Australia are. And today's Federal Republic of Germany maintains the formula of a federal state, in which case the nation is composed of states with inherent jurisdiction and the federal government's powers are only those that the states have given to them.
There is a line of thought among some that the deposition of German and Austrian monarchies in 1918 was not a good thing, that the perceived vacuum left the door open to totalitarianism. However, Kaiser Wilhelm III was very much the culprit in this war, so the loss of the throne was no surprise. Perhaps there is certain merit in this argument, but what there are a few things that are certain:
- many in Hanover never accepted the loss of Hanover's independence in 1866. From then until WWII, there was a political movement with Reichstag representation dedicated to restoring the state of Hanover.
- similarly, in Bavaria, there was and to a degree still is a separatist movement. Separatism and monarchism were fairly strong sentiments in Bavaria during the Weimar Republic. Bavarians always saw themselves differently from the rest of Germany, culturally they have many things in common with Austria. Austrians and Bavarians can be said to form an ethnic subgroup, while Alemannic people (i.e. German-speaking Switzerland, Liechtenstein, Voralberg, Swabia and Baden-Württemberg) form another.
- there is no doubt that in a few German states, and most notably Bavaria, many and perhaps a majority of people have never accepted the their royal houses being deposed. Certainly the Wittelsbachs are still highly respected in Bavaria, and the Württembergs seem to be as well.
The influence of Germany's aristocratic class, despite the derecognition of any and all titles, remained strong long after, and that includes having at least one President of the Federal Republic- namely Richard von Weiszäcker, who was also President at the time of reunification.