Sorry to be a pedant but BER already exists. It is the 3 letter code for the metropolitan Berlin area. I.e. the code designates any and all airports for Berlin. Each Berlin airport has its own three letter designator (TXL, SXF)

We won’t know, until it opens, what the IATA or ICAO codes will be.

Three letter codes are in (very) short supply and IATA doesn’t like to waste them.