Game of Thrones is an Amaizing series of books and also a very entertaining show.
There is still so much unknown about the Lore, Mythology, Magic System, Class features etc etc. Hell...even White Walkers are still a small mystery leaving out such detail.
Try the PC games. That might float yourself some ideas.
There is so little to be known that anything made by non professional sources (PC games, Goerge RR Martin approved resources etc etc) will be half done at best filled with Guesswork.
I'm sticking with Pathfinder and GURPS.