Hybrid systems open up many possibilities. Form efficiency, weight and balance, to a redundancy for safety. At work I was working on an R&D Project using some large Brushless DC motors. Just for kicks, I connected one motor directly to another and I was amazed at there efficiency. As I turned one motor the other turned at the same rate. While there was some loss of torque transmission the efficiency between the generation of power and the output from the motor was mind blowing. I think for instance the PSR unit on a Rotax engine could be replaced by generator/motor system and would operate more like a constant speed prop with a battery for redundancy with out much weight penalty.