A Walmart would be not too different from the Communist palace there previously, but there should be something noble on the site to match the neoclassicism of the Museums, in my humble opinion. I love the rebuild of the Frauenkirche in Dresden and look forward to the restoration of the Neumarkt area around it, although I think all of that is more thanks to donations and private funding than public funding. Couldn't they just have a private developer build something on the Stadtschloss site and require a halfway decent facade with a gesture or two to the Stadtschloss tradition?
Of course nobody is going build a Walmart there (just my form of sarcasm), but I also doubt that rebuilding the Stadtschloss of the Hohenzollern, the family of Wilhelm II, is a good idea. In difference to the Frauenkirche in Dresden, which reflects the will of the German people to rebuild a shattered temple for our Lord, the Stadtschloss does not bear this deep symbolism. We all know that the last emperor was a idiot and in parts responsible of the first world war with his childish war games. The former "Palast der Republik" of the Commis had at least the advantage that it represented real history. Anyway it was ugly.
Therefore I could imagine a modern building in high quality there. To "quote" (I do not know how to express this in English) just old architecture is eclecticism and thus boring. Sometimes we have to invent ourselves new.