Granger,
The single rotor configuration may have reached its limits, particularly in respect to forward velocity. Small improvements will be 'tweaked' out, but no quantum leaps IMHO.
tgrendl,
Intermeshing, coaxial or tandem. Any configuration where the two rotors aerodynamically interact.
The biggest disadvantage of overlapping rotors is vibration. As technological improvements continue to be made in eliminating vibration (Higher Harmonic Control, Individual Blade Control, etc), it is the helicopters with overlapping rotors that will see the greatest benefits.
More coffee.
This thread is a request for all 'wild & wacky' ideas. Thanks.