Surely, in an ideal world, ATC would no longer exist (at least not in the form we know today). Each aircraft would perform a "perfect" flight plan without having to fly from beacon to beacon at a fixed height. They would fly the great circle at optimum altitude. To achieve this you'd need both air and ground based systems (I suspect) with frequent position reports generated automatically from the aircraft and the ground systems calculating where to route the aircraft for avoidance purposes and aircraft based systems along the lines of TCAS to deal with unexpected problems.
I love science fiction!
We are actually moving in that direction, not sure how far along it is, but there are moves to do away with airways in Europe, and you
will basically be cleared great circle track from dep to destination, though level will for some time be assigned, a/c will need to be CPDLC and, I think, have sat nav.