Well Kit Harrington himself said it was cancelled because they "couldnt find the story" so bascially they were just gonna make a show off of game of thrones popularity and the fact kit harrington isnt batting off offers from all over hollywood and has a free schedule.
I think a cool story post game of thrones would have to be the prequel of the rebellion war. Done right its got all the elements of action, big characters, twists and turns. And would only add to the legacy if it was good because then by the time you start game of thrones with Ned stark etc the character would be even more richer and his end more tragic again.
Just like they should do a first age of the lord of the rings