The creator of the world of Game of Thrones, George R.R. Martin, doesn't know how the TV series will end. Martin's book series A Song of Ice and Fire is the basis for the popular fantasy show. For the first handful of seasons Game of Thrones followed these books closely with each season being dedicated to adapting a single novel. Martin was even involved with the production of the show writing several episodes and having a cameo in the show's original pilot.
Starting with season 5, the series began to pass the books, introducing storylines, twists and character deaths that have not yet occurred in the source material. Game of Thrones will end well in advance of the book series, which has yet to receive a release date for the sixth book (out of seven). As such, even though Martin created the overall plot structure of Game of Thrones and has an ending in mind for the books, the TV series could be wildly different. Martin will learn how Game of Thrones ends with the rest of the viewing audience.
Martin revealed that he's in the dark about Game of Thrones' ending in an interview with EW. The reason is rather simple. He's hard at work at completing the sixth book in the series Winds of Winter. This isn't exactly new either; Game of Thrones' D.B. Weiss explained that Martin stepped away from the series during season 4 to firmly separate the show from the books in his mind. However Martin has been so devoted to getting Winds of Winter finally out on book shelves that he decided not to read any of the scripts for the final season or even film a tiny guest spot before the finale. The author explained:
I haven’t read the [final-season] scripts and haven’t been able to visit the set because I’ve been working on Winds. I know some of the things. But there’s a lot of minor-character [arcs] they’ll be coming up with on their own. And, of course, they passed me several years ago. There may be important discrepancies.
Since Game of Thrones began based on a book series that was still unfinished, Weiss and his fellow showrunner David Benioff did meet with Martin before the series began. At that meeting they received a rough outline of the ending from Martin. The finale of Game of Thrones will be based on Martin's original ending for the series as it existed circa 2011. However Martin has been open about discovering plots, characters and developments while he is writing the series. Game of Thrones could end exactly as Martin first envisioned his story coming to a close, but the books could reach altogether different ending when Martin completes them.
Even more importantly, Game of Thrones has been a lot less restrictive when it comes to following the books in recent seasons. Sansa Stark's controversial marriage to Ramsay Bolton never occurs in the books and instead Ramsay marries (and abuses) a very minor character instead of the eldest daughter of House Stark. This one change had an enormous impact on Sansa's story moving especially how it related to the death of Ramsay in season 7.
It might be better to look at the show's happier changes though for how the books and show will deviate. The romance between Missandei and Grey Worm, which is prominently featured in the show's final season trailer, is just as invented for the series as Sansa and Ramsay's marriage. It's hard to know exactly who Martin considers a minor character in Game of Thrones since the cast is so vast. However it is safe to assume that when Martin refers to the series changing minor character arcs the ending of Missandei and Grey Worm's story is one of those examples. Though it wouldn't be all that surprising if Jon Snow's story ends in a vastly different place in the show than it will in the books.
Game of Thrones season 8 premieres Sunday, April 14 on HBO.