The Advancing Blade Concept may make a comeback. The S-69/XH-59 with
stacked co-axial rotors was hampered by empty weight and hub drag.
Today's improvements in composite materials, fly-by-wire controls, and
transmissions may make the ABC viable for higher-speed applications.
Sikorsky is again showing ABC concepts, so . . .

The telescoping tilt rotor idea is still around, but I somehow see that
less likely than the ABC.

HW