I'm kind of keen to see if large electric cargo motor gliders might one day become a thing. Traversing great distances via ambient energy harvesting. Maybe even self landing at certain designated airfields to top up on energy and avoid bad weather.
Most of the stratospheric approaches I've seen aren't so much about exploiting low altitude weather phenomenon but rather flying above it. Which of course is exactly what you want for long term remote sensing.
I'm thinking systems that mostly exploit thermals and updrafts, engaging in a kind of bird like automated soaring.
A migration of the machines so to say.