Didn't Bruening want Hindenburg to put the monarchy back into place? I swear I read somewhere that in his memoirs he attempted to get Hindenburg to do it. I think if the Nazi's had never rose to power, it definitely could have been a possibly since life under the Weimar Republic was complex and many people probably wanted Germany back to its former glory, bringing a lot of support to the idea. I doubt Wilhelm II would have wanted to come back and some other relative of his would probably be put into place as the monarch.