Stadtschloss??! Berlin??! BuHuHaHa!
This will never be done. Berlin is completely bankrupt and nobody else in Germany would spent a single cent on it. We already wasted too much money in our capital. Besides - in my opinion it doesn't make any sense to rebuild someting nobody really knows how it looked like. If they are really going to build this they also could build Cinderellas castle on this place. I predict that this area will be a parking place for the next twenty years until somebody is going to buy it for building a nice Wal-Mart on it.
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?