A single wing on one side of the helicopter would produce another (roll) problem in the hover from downwash.
I think we have a good compromise with the present designs, which have evolved over a number of years into a good old engineering and aerodynamic compromise. I remain still to be convinced by the tilt-rotor concept, which is too specialised and so inherently relatively inflexible. I think a better way forward would be to resurrect the Gyrodyne/Rotordyne concept.