Could build it on the old airport.
It was originally intended to replace both Schönefeld and Berlin Tegel Airport and become the single commercial airport serving Berlin and the surrounding State of Brandenburg, an area with a combined 6 million inhabitants. However, it is now planned that it will not replace any, with Schönefeld Airport currently being expanded due to rising passenger numbers, and Tegel which will remain open after a referendum.
After almost 15 years of planning, construction began in 2006. Originally planned to open in October 2011, the airport has encountered a series of delays and cost overruns.
Since German reunification, air traffic in Berlin has grown greatly. In 1991, the combined passenger volume of the city's airports was at 7.9 million per year. By 2014, this number had risen to 28 million. When Berlin Brandenburg opens, it will have a capacity of 27 million passengers per year.
Summarized: when/if BER will open, if it will manage to handle immediately its max capacity (best-case scenario), it won't have enough capacity => the other airports will still be needed (until BER is expanded) :|