Despite the countless Game of Thrones spinoff rumors and announcements, it appears that only one spinoff, a prequel produced by Jane Goldman, is currently being developed. The spinoff is set to air after the conclusion of the main Game of Thrones story.

Since 2011, the adaptation of George R.R. Martin's original story has skyrocketed in popularity. While the final season of the original show is set to air in the first half of 2019, it is hard to imagine that HBO would not be looking to further capitalize on the story of Westeros, especially with so much rich history and lore to explore.

In an interview with EW, Casey Bloys, president of programming at HBO, said he does not "anticipate any movement on any other prequels until we [see what this one is]." Bloys also said that they are currently hoping to begin shooting the prequel in the first quarter of 2019, which would coincide with the airing of the eighth and final season of the main series. This would give fans of the series something to look forward to until 2020 when the new series will likely air. HBO released the following synopsis about the upcoming spinoff:

"Taking place thousands of years before the events of Game of Thones, the series chronicles the world's descent from the Golden Age of Heroes into its darkest hour. And only one thing is for sure: from the horrifying secrets of Westeros's history to the true origin of the white walkers, the mysteries of the East to the Starks of legend... it's not the story we think we know."

George R.R. Martin has suggested the title "The Long Night" for the prequel pilot, but Bloys chose to not reveal any possible working titles of the prequel. Bloys has also stated that the 8,000 years between the prequel and the main series is enough time to give the prequel its own identity, especially since the kingdoms of Westeros do not exist at this point. 
It seems as though this is an intentional effort by HBO to distance itself from the main story.

"We're not trying to do Game of Thrones Part II. No one is going to duplicate what [GoT showrunners David Benioff and Dan Weiss] did. By setting it when we did, there's a lot going on, a lot of dynamics that are related to Game of Thrones, but it's different enough with its time period and characters its duplicative."

With the success of the original story, showrunners will have their work cut out for them if they are looking to create their own story as grandiose since audiences will not easily disassociate the lands of Westeros from the more significant characters in Game of Thrones.

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With Jane Goldman on board the production, who has written and produced a variety of successful titles such as Stardust, the Kingsman films, X-Men: First Class, and Kick-Ass, and the large budget being allocated to the production, fans might prefer that one quality spinoff be focused on, rather than multiple stories that may fail to live up to their expectations.