Riff, agreed.
For fixed wing there is an arguement for hybrid, with engine (or Intelligent Energy fuel cell ) providing take-off or go-around power. Helis max power requirement is usually in the hover, so it needs to be sustained.
Now that i'm involved in diesel engine design, i can see the difficulties with many engine concepts targeted at aircraft. Aero emissions isn't as tight as for their ground vehicle cousins, but no need to introduce problems like inconsistent scavenging and combustion. I would not be suprised if developments involving turbine LP stages and piston HP stages enter discussions soon. Actually, i would keep an eye on the humble flywheel.
With new rotor technologies, like Blue Edge and Blue Pulse, improving machine efficiency i would be interested to see how disk loading evolves in the coming years.