When it is built, it will take all tram traffic in Barbarossaplatz underground. According to  Kölner Stadt-Anzeiger, The inner-city tunnel will be extended from Barbarossaplatz to the justice center on Luxemburger Strasse (line 18). In addition the ring tunnel (lines 12 and 15) will be extended beyond Eifelstrasse and line 9 is to be moved into a tunnel.

If and when built. Since it was announced in 2009, for so many years there is no concrete progress. There was a city council commission for a feasibility study in 2020 which is yet to start. According to news, nothing will start until 2026 with no guarantee of if it will start then either.

https://www.ksta.de/koeln/koelner-innenstadt/neustadt-sued/koeln-drei-neue-u-bahn-tunnel-fuer-den-barbarossaplatz-799358