I can't help but feel that the diesel/Jet-A1 piston aero-engine is going to be a short-lived beast. More likely in the very light range it'll be overtaken by pure-electric aircraft (as we have already seen discussed on this forum) and in the larger (longer-range) arena someone will come up with a neat electric-turbine hybrid. Little turbines like the Solar T-62T-32 are already being used in a bunch of small helicopters (as well as in larger aircraft as an APU), they're relatively affordable, and with an electric drivetrain you can handle short-term loads and speed changes without upsetting the turbine.