Desperately Seeking The Next...
It's a bit presumptuous of me to have a strong opinion, since I only watched a couple of episodes, but hey, presumptuous ought to be my middle name. In my defense, I did read four of the books before I gave up after concluding that (a)the series wasn't getting anywhere, or at least not fast enough to hold my interest, and (b)George R R Martin was a sadistic sicko.
But back to the topic: what were the secrets of the success of GOT? My guess: sex, violence, nudity, sadism, and a multithreaded plot of enormous scope. And dragons. And other monsters.
Of course nobody should be confident that that's what the viewing beast will want next time, but those are timeless themes. My favorite set of books in the SVNSP department is the John Carter of Mars series, though unfortunately it has already been made into a fairly unsuccessful movie. Maybe because they skipped the nudity.
HBO and it's big time series have made it clear that television is a medium that allows one to tell stories of more than novelistic scope. In effect, they have invented a major new art form. But how to get the needed content?
Long novels, or novel series, offer a promising target. Some provider, Amazon I think, has coughed up something like a quarter of a gigabuck for Lord of the Rings. Promising? Maybe, but it is hard to see how they fit in the sex and nudity. Ditto for Asimov's Foundation series.
I suspect though, that there is some fantasy series out there ready to make the grandchildren of the author fabulously wealthy.