I'll preface this by saying I have not played Game of Thrones but I've recently been tinkering around with Crusader Kings 2 and have played plenty of Mount & Blade over the years and while neither are technically astounding, a combination of the underlying mechanics seem perfectly suited to the books.
Political intrigue, espionage, familial alliances as well as alliances of convenience (which quickly fall apart). Mercenaries, specialists and the whole beautiful, bloody mess of siege warfare that Mount & Blade encapsulates seems ideal. Add in the robust political and lineage based strategy of Crusader Kings and it could be a great game. Though sadly complicated and not particularly marketable.
I still think if Taleworlds and Paradox Interactive AB got together they could make a helluva game, even if it lacked the marketability of a licensed brand.