Pretty hard to do in a dense city and doesn't help very much. Unless kill Berlin's character and transform the city center into a collection of skyscrapers, demand for the city center will always far outstrip supply. Berlin is not San Francisco, where large parts of the city center are single family homes with two stories or less.
Berlin really isn't that dense, and it's surrounded by Brandenburg, which is a whole bunch of flat, empty land. Furthermore, the rail network exists to handle extra capacity, so there really are no more physical constraints on building further.
