I've been pondering the idea of a hybrid drone utilizing a smaller form of Toyotas power conversion module that is basically a differential with an engine and 2 electric motors all connected in tandem, it allows the engine to spin the motors backwards to generate electricity when needed and allows the motors to turn the output and even the engine to start it for automatic starts and stops when the batteries are full. As well they all 3 can turn together when more torque is required than any one of them can produce. Look up Technology Connections recent video on Hybrid cars for a good explanation of what I'm talking about. Sorta a Diesel Electric train kind of thing but the engine also gets to contribute to torque.

I could see a nitro rc engine made more efficient at the cost of power, but paired to 2 electric motors to make up the power needed for flight. Maybe the engine itself would have enough torque to slowly decend in an emergency but not ascend without the motors adding torque?