True, engine efficiency is kind of flat-lined. Turbines aren't efficient for most GA operations, but there are new diesel aviation engines coming online.

Aerodynamically, I don't know about big gains. Something like a Lancair Legacy is pretty low drag. You can make lower-drag planes, but that typically comes with much higher stall/takeoff/landing speeds that make the plane harder to fly and much less safe in the event of a forced landing.

Personally I'm looking forward to the next generation of electric aircraft, once battery density improves and costs come down. A Chinese company is already producing an electric ultralight and working on a small GA electric: http://www.flyingmag.com/aircraft/lsasport/germany-certifies...