The lid should have a solar panel on one length/width plane and a screen on the opposite. There should also be space for WiFi (and maybe LoRa) antennas - an external connector for LoRa could be added later, like an antenna dongle.
The spellbook is cool, but could you replace the back panel with a solar panel?
Could you replace the RPi with a RISCV board, like a HiFive1?
Could you find a laptop/netbook size/grade eInk display?
Could you write a custom operating system for it in Rust? This one's a long shot, but it would be a lot of fun.
The HiFive1 is probably too low-power. The 32M flash memory could be overcome with SPI flash or an SD card, but 16K of memory is probably nowhere near enough. It also doesn't have a GPU, but you could hook up a display.
More RISCV processors are likely to appear, though.