Designing a solarpunk world. I wanted people living within the ecological limits in every biome: arctic, tundra, jungle, sailing the sea, sailing the sky in airships, under the sea in submarines. This last one posed a massive problem because submarines are extremely power-hungry, and that's incompatible with the low-energy-use theme of the world. But I didn't want to give up the Zissou vibes by erasing the submarine-tribe.
The existing ones are small drones. But a paper with (doi:10.1007/978-3-319-16649-0_12) has a section titled 'Size effects' and bigger would be better. And the paper already linked has a whole chapter discussing scaling effects.
That was a great breakthrough because I went from "Submarines are poewr-hungry nuclear behemoths" to "Submarines use basically zero energy" and did it with proven tech. The tradeoff is that you have to glide up and down, and the floor will be at a 4° tilt a lot of the time, which could be annoying.