"I do have thousands of pages of fake history of everything that led up to Game of Thrones, so there’s a lot of material there and I’m writing more," Martin said backstage after the show won the Emmy for Outstanding Drama Series, Deadline reports.
That said, Martin was careful to point out there are currently no official plans for a prequel, noting that "at the moment we still have this show to finish and I still have two books to finish so that’s all speculation."
In the event that a spinoff series does happen, showrunners David Benioff and D.B. Weiss said they won't be involved in a prequel. "It’s a very rich world. I'm sure there will be other series set in Westeros," Benioff said backstage at the Emmys (via TVLine). "But for us, this is it."
http://www.ign.com/articles/2016/09/19/game-of-thrones-george-r-r-martin-discusses-possible-prequel-has-written-thousands-of-pages-of-lore

I could understand if he was doing it simultaneously to help himself as continuous background to help him write the main stories, but otherwise...priorities man! Maybe he'll really start taking it seriously once the show ends.Game of Thrones: George R. R. Martin Discusses Possible Prequel, Has Written Thousands of pages of History
I sure am glad he's taking time to write thousands of pages of fake history that probably will never be published entirely. It's not like he has anything better to do with his time.![]()
Would you guys recommend the Dunk and Egg novellas?
I could understand if he was doing it simultaneously to help himself as continuous background to help him write the main stories, but otherwise...priorities man! Maybe he'll really start taking it seriously once the show ends.
Oh right, ignore me then lol.Most of the stuff he's talking about is previously published material.
-" It’s funny, I’m not really sure what happened there. I was contracted to do at least four episodes this season, but then I was in L.A. doing publicity for something else, and I got a call at the Chateau Marmont and there were familiar voices on the other end of the phone. It was one of those guys, and because they didn’t introduce themselves it was like, “Hi, it’s me.” I was like, “Is that David or Daniel?” Anyway, they said, “You know what this phone call is about.” I was like, “Yeah, well, I guess I do.” “So we were going to kill you off at the end of last season, but we decided that we’re going to have to kill you off at the beginning of next season.” I was like, “Okay, life goes on.” But there was something wrong about that because I had been contracted for four episodes in the following season, so if they were going to kill me off at the end of the last season why would they contract me for those four episodes? Because it costs them money whether I do them or not, so it’s not great business sense to do it just in case. So something happened; I have no idea what. There was an enormous amount of fan excitement when I got named to be on the show, and everyone was like, “Oh my god, yes, Doran Martell. He’s going to be great as Doran Martell.” That might have been the kiss of death. Maybe they didn’t want quite that much attention on that character. Maybe they thought, “Well, let’s prove that we’re going to stray from the books. We’re going to do something else, and he will be our first example of that.” So maybe that could have been the case. Or maybe I just screwed up. Maybe I said the wrong thing to the wrong person."
That pretty strongly implies that their endgame was always to have Doran's place usurped by the Sand Snakes, and that they valued that outcome more than Siddig's appearances as Doran being almost unanimously preferred to the Snakes and Ellaria. So either Doran is doomed to a fairly ignominious ending in the books, or they made the wrong long term decision in scripting the show and couldn't break from their path.Alexander Siddig on GoT season 6:
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That's very odd, but I guess they drastically rewrote things after the backlash Dorne got in the previous season.