Aura HD hack ability has been mentioned and is really first rate. While hacking the new Aura will improve, the HD has removable internal SD card that lets you upgrade internal memory and easily install saved image of known good firmware to internal memory so anyone can recover from bricking their hacked Aura HD reader. New 7" Aura memory is soldered to circuit board.
Kobo Developers Corner
https://www.mobileread.com/forums/forumdisplay.php?f=247
Has threads on installing different reading software onto Kobo readers and adding self help improvements to Kobo firmware. While other companies try to prevent hacking of their readers Kobo doesn't.