Adalbert of Prussia, Wilhelm II's Naval officer, relatively normal, third son acting as Regent for a nephew?
I always found it slightly strange this wasn't considered as an option in November 1918. He had a respectable career, was a respectable guy by the standards of his time and class and had enough training to understand his political value as a stabilising force to ease Germany into something different but still recognisable whilst accepting constitutional limits akin to the Windsors whilst binning his Dad and brother to the Netherlands regardless to save the dynasty. He could have been a useful diplomatic tool for Germany to try to rebuild relations with their neighbours especially with the French and the Poles and get financial support for repatriating Volkdeutsche to Germany proper. This option is less traumatic and extreme and limits the 'stab in the back' myth's potency early on. Basically Ebert could have his cake and eat it.
I believe Adalbert's daughter married a random American and one his grandsons sold shoes in California. That's quite a felled dynasty. Adalbert is also one of Queen Victoria's MANY great-grandchildren, as is George VI. Using him either during the 1940s or in the Interwar period through his family relationship with George V/VI to exploit UK ambivalence about Versailles Interwar or concerns about post-war settlements during the war and look at other options would have been smart of someone like Stresemann or someone anti-Nazi mid-war too. Who knows, the Corporal might have been, should have been avoidable by using what was already there but not personally compromised to buy time until young Wilhelm (the one who died in the Army in 1940) came of age in 1928 trained for a different type of monarchy, sort of like Charles, Count of Flanders did in Belgium for King Baudouin after the Second World War.
Otto von Habsburg is probably the really obvious one, being inventive and broad in your definition of 'Germany'.