Always thought that a single engined pusher was the ultimate WWII fighter.
I would have used a radial like the P-56 with a cooling fan. These were already used in tanks powered by radial engines so they weren't new. Simplicity of the radial with the aerodynamics and reduced frontal area of an inline. Tricycle gear, heavy nose armament, pilot out front for better visibility, big fuel tank in fuselage over the CG. No propwash to fly through. Flying wing, canard or twin boom tail, it could have been done and would have been very efficient.