It will not be an easy one to solve , because waiting 10 or 20 seconds doing nothing when aircraft are on a collision course is not what controllers were and are trained to do.
I am not advocating "doing nothing." However, there are other things an ATC can do RIGHT NOW without ANY change in TCAS logic:
Call a turn.
Ask the Pilot if he is monitoring TCAS. TELL the Pilot to monitor and follow TCAS.
Either of those would have prevented the mishap at Ueberlingen. Why are we so focused on automated intervention in the vertical plane only, when in a controlled environment ATC also has the option of using the horizontal plane? In many cases the pilots are not aware of the other airplane's actual or projected heading/course, so they cannot independently turn. However, ATC has a clearer picture of the collision intercept, and is better able to assess the best turn geometry.