Sounds to me like it might as well be original fantasy, except for maybe the occasional hint to an established location, story or family name. I think most of the benefits of making it a "spin-off" is on HBO's side, namely, people who are fans of GoT will check it out and you will therefore have a build-in fanbase already, and you can get away with making a show that's pretty similar without being called out on ripping off GoT.