I am reading on Entertainment Weekly that the spin-off is likely chronicling the end of the Age of Heroes, i.e. Bran the Builder, Lann the Clever, the beginning of the Long Night, etc.
http://ew.com/tv/2018/06/08/game-thrones-prequel-jane-goldman/
Jane Goldman the likely writer with big George providing good input and guidance.
Really excited about this narrative if it comes to fruition, its not just going to fill in gaps of the story currently on screen because its set 10000 years before, but its status as myth and legend in the current storyline makes it intriguing and relevant