> Jumbo jets won't be electrified anytime soon. Weight (and thus, energy density/kg) is everything. But synthetic fuels, hydrogen etc might be good options.

That's making me think that hydrogen-filled airships (Zeppelins/dirigibles) might be practical with some kind of electric propulsion. That way, the weight is no longer so much of an issue, though the trade-off is that they'll need to be bigger. Their speed (or slowness) could be an advantage in that they should be much easier to fly and collisions would hopefully be infrequent and not so catastrophic (I'm picturing more of a bounce than a collision).