How many legitimate stars will come from this show? Sophie Turner, Kit Harrington, Emilia Clarke, Peter Dinklage (although he certainly deserved to be one before this) are the ones that I see.
Definitely Sophie Turner, I really think she has the best chance, tall, young, beautiful and talented. The others ... nah. Harrington will get a few roles but will be tossed aside for the next hot and young guy that comes along, Clarke can't act and fucked up big time by starring in Terminator genysywhatever and Dinklage is too small, like it or not but he can only play specific types of roles, he'll work but more as a character actor.
I think Lena Heady will get more and better roles, she's already an established actress and GoT helped raising he profile but she's too old to become a major star at this point. She should be but the roles a woman her age should play usually go to starlets in their early 20s, that's how 22 year old Kate Bosworth was cast as experienced Pulitzer Price winning journalist Lois Lane who also had a five year old son. Not that Lena should play Lois but it was exactly the type of major role that should have gone to an older actress but didn't. Happens all the time, they finally got it right with Amy Adams as Lois, the movie might have sucked but the casting was great.
I think many people will say Maisie Williams but I don't see it, people are in love with her character and that will allow her to work in genre shows at least but beyond that ... I just don't think it's going to happen.