I see hybrid (electric/diesel) propulsion as a great way of minimizing environmental impact. Use electrics to boost climb, use diesel/Jet-A for the cruise. Hovering is never energy-efficient, so STOL aircraft, where half the power comes from electrics and the other half from the reciprocating unit(s).
Supercapacitors are a great alternative to LiPo batteries and do not age the same way!
For smaller planes, a folding prop, and an electric powerplant in the nose, and a pusher unit in the rear, powered by a diesel engine would probably be ideal!