I like carrot cake - and most of all I like the icing on top of it. The combination of the creamy softness of the icing and the moist carrot cake itself. Delicious!
Yet, I would much prefer the reconstruction of the Berliner Schloss without icing, since it is not a carrot cake but a building of the 21st century.
No icing please! Leave it as is: Plain and simple concrete. Just put some equally plain and simple stainless steel windows in.
And then let it become the Humboldt Forum, as a lab for future generations.