There's definitely more to be had in ICEs especially combined with hybrid transmission systems however the perpetual issue is that even expensive fuel is very cheap compared to the capital cost of trying to save it. Currently ICEs are being asked to span too large an envelope to be really as efficient as they could be. Also the current trend in highly boosted engines is less Otto cycle and more combined cycle where multi-stage compressio and expansion is used to improve overall cycle efficiency in the same manner as gas turbines do.
Electric and hybrid-electric aircraft are not really about being green as much as they are about opening up new design opportunities, particularly in the VTOL space. This comes largely from the ability to precisely control motor power electronically as well as the chance to have high power density motors in aerodynamically convenient places. This is new territory though and no one really knows what an optimal configuration actually looks like yet so you're going to see some weird contraptions proposed until we learn enough to know where the advantages lie. All of the electric motor technology is already highly developed and more-or-less on the shelf waiting to be adopted
As far as batteries go, sure there are likely gains to be had out there in the future but useful things can be done with current technology as Tesla has demonstrated in the automotive sector. Whether you like the results or not is irrelevant. The product is as compelling as a phat S-class Benz ever was and as justifiable. Forget the green tripe and look at it purely as a luxury car buying option.
Like any design problem there is no one solution and its all about pushing the compromises around until you get something that works well (or not). Over time designs will converge on perhaps one or two configurations and the real effort becomes fine-tuning the details
Last edited by Andy_RR; 12th Jan 2019 at 00:21.