If engine weight is allowed to increase, i'm suprised there isn't a renewed effort to investigate turboshaft engines with regeneration. This ais used commonly in ground based power turbines, although heat exchanger mass is the limiting factor for aircraft. It would also be possible to design the optimum efficiency for medium power setting, with reheating and intercooling used to provide hover power requirements.
See page 6 of the following:
http://files.asme.org/IGTI/101/13001.pdf
More general info:
http://www.energysolutionscenter.org...ombTurbine.htm
http://web.me.unr.edu/me372/Spring20...generation.pdf