Long-time series writer/producer Bryan Cogman is at work developing a fifth Game of Thrones prequel/spinoff show for HBO. It was first reported back in May that HBO had plans to perpetuate the Game of Thrones franchise beyond the main show by crafting one or more new series to be set in Westeros' mysterious past. Game of Thrones showrunners David Benioff and D.B. Weiss will not be directly involved in any of the planned universe-expanding spinoffs.
Initial reports said HBO was developing four Game of Thrones spinoffs, but president of programming Casey Bloys later clarified that it's possible only one of the four series will ever be greenlit. Details on exactly what pre-Game of Thrones events would be covered by the prequel or prequels have been kept under wraps, but fans have been happy to offer up suggestions in case HBO is looking for ideas.
EW now reports that an additional Game of Thrones spinoff series is being developed, bringing the grand total to five. Long-time Thrones writer/producer Bryan Cogman is the man developing the fifth series, and he reportedly has worked closely on it with George R.R. Martin himself. Soon after news broke of the four in-development prequels, Martin referred on his blog to a fifth series that was also under development, but HBO never confirmed the existence of this fifth spinoff series. Details on this series are naturally being kept secret.
HBO brought in a number of outside writers, including Jane Goldman (Kingsman) and Max Borenstein (Kong: Skull Island), to begin developing their four previously-reported spinoffs. Cogman differs from those writers in that he is a veteran of Game of Thrones, having written some of the series' most popular episodes including season 7's "Stormborn", season 4's Tyrion-on-trial episode "The Laws of Gods and Men" and season 3's "Kissed by Fire".
Cogman's intimate first-hand knowledge of the Game of Thrones universe could give him an advantage in developing a spinoff series that clicks with HBO brass. It also doesn't hurt that Cogman has apparently been picking Martin's brain during the process of working up his idea. No one knows what Cogman or anyone else is planning as regards the prequels, but Martin has ruled out a few possibilities, including tackling Robert's Rebellion, the huge war that largely set up the events of Game of Thrones season 1.
HBO has plenty of time to sift through all the various ideas and pick the best one, as the spinoff/prequel will reportedly not begin production until at least a year after Game of Thrones season 8. Season 7 recently finished airing on HBO and it garnered some of the biggest ratings in the show's history.
Game of Thrones season 8 premieres either in late 2018 or early 2019.