Fellow Cohostians, please accept this offering to Eggbug: the RAPU (Ridiculous Aviation Propulsion Unit)
The idea here is a radial engine mated to the fan (or first compressor stage) of a modern high-bypass turbofan.
A power-recovery turbine uses the radial's exhaust gases to turn both a supercharger and an electrical generator. The supercharger is coupled to the PRT via a planetary gearset with the ring gear being driven by an electric motor, allowing infinite gear ratios. Its purpose is to maintain manifold pressure for the radial engine at altitude.
The generator provides current to a high-frequency inverter and high voltage step-up transformer, which in turn drive a magnetron that ionizes the bypass air into plasma, increasing thrust.
The inner nacelle serves several functions, guiding cooling air from the ducted fan around the radial engine, housing intercoolers for the supercharged intake air, and oil coolers for engine oil.
Red is exhaust flow, blue is intake airflow, and orange is plasma flow.
The image was made from a cutaway drawing (not mine) of the Wright R-3350 radial engine super-imposed over a cutaway drawing (not mine) of the Pratt and Whitney PW1000G high-bypass turbofan. The idea for the magnetron plasma thruster came from real research (not mine) into air-breathing plasma thrusters.