I wonder if the semi-circular / quadrangle flight level system would have prevented Überlingen? Was the lack of this feature a failure to ‘defend in depth’, - over reliance on radar systems, need to handle more aircraft in a crowded airspace, or the human desire to help others by giving them direct routing?
Yes, Überlingen would have been prevented, if both crews had made a semi-circle to the right (or the left). If promoting this to a standard resolution to a collision threat though could be problematic in crowded airspace. Note that the issued TCAS Resolution in the Überlingen cause itself would have prevented the collision - as would have the descend order to the Tupolev issued by ATC.
ACAS III was planned to implement horizontal RAs.