Berlin is a capital without industry and banking and has attracted only a few corporate HQs. The federal structure of Germany and the fact that many government officials remain in Bonn are other factors that reduce the importance of direct longhaul routes to Berlin.

Even with the new airport I would expect not much change as traffic flows are already quite established in Europe. Maybe there will be an influx of LCCs and holiday charter carriers but I wouldn't expect too many new longhaul connections to materialize.

And if FRA and MUC should become overcrowded, there are many alternative hubs for German pax to choose apart from Berlin. AMS, CDG, CPH, HEL, LHR, VIE, ZRH...