Given that the Age of Heroes on Game of Thrones and in the A Song of Ice and Fire book saga was around 8,000 years before the events of Game of Thrones, we shouldn't expect any of the key Thrones characters to reprise their roles unless the spinoff goes the route of Frodo appearing in The Hobbit. That said, Bran Stark could have a part to play, especially if theories that he uses his abilities as the Three-Eyed Raven to travel back in time and construct the Wall as Bran the Builder are accurate. Isaac Hempstead Wright could have a part to play, and it's not out of the realm of possibility for some of the Game of Thrones actors to play their characters' ancestors. If early Starks and early Lannisters appear, perhaps some of their Game of Thrones actors could play them. One actor is already on board should the opportunity arise!