Ventus45 said "Where to from here?”
ADS-B.
The issues of equipment redundancy still apply, but a collision avoidance system based on ADS-B would be infinitely superior. The accuracy is such one could do lateral escape maneuvers.
As we all know, TCAS is far from a clean sheet of paper design. It is a (for lack of a better word) bastardized system added on to a transponder design not envisioned to be an in-flight collision avoidance system.
With a CDTI (cockpit display of traffic information) and ADS-B, each aircraft would have "seen" each other over 100 miles away.