If I could give my two pen'orth as a powertrain engineer, the current Lycoming / Continental engines with carburetors, magnetos, manual mixture and intake air temperature controls are ridiculously out of date.
If one could afford a clean sheet, the best compromise would seem to be:
compression ignition (less to go wrong, low speed torque characteristic, fuel consumption, no throttle icing because no throttle),
air cooled (less to go wrong),
current technology diesel fuel injection pump, injectors and control equipment (to give modern standards of starting, handling, consumption and emissions),
dual fuel (road diesel or jet fuel),
flat four (to fit the prevailing aircraft nose and nacelle shapes),
modern materials (you don't expect your road vehicle engine to break when it gets a shock cool as you drive through a ford)
If someone wants to front me the cash, I’ll run the design and development programme for you!