If early Starks and early Lannisters appear, perhaps some of their Game of Thrones actors could play them.
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.
One actor is already on board should the opportunity arise!
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.