In a recent interview with Entertainment Weekly, Michael Lombardo, HBO's programming president, announced that Game of Thrones would run to at least season eight. Originally David Benioff and Dan Weiss, the showrunners, had declared that season seven would be the end. However now it looks like that isn't the case at all. Lombardo said this:
Seven-seasons-and-out has never been the [internal] conversation. The question is: How much beyond seven are we going to do? Obviously we're shooting six now, hopefully discussing seven. [Showrunners David Benioff and Dan Weiss are] feel like there's two more years after six. I would always love from them to change their minds, but that's what we're looking at right now.
Of course this doesn't mean the show has been green lit for the eight seasons. But it is reassuring to hear that the option is now being talked about.
In the interview with Lombardo, he was also asked about the possibility of doing a prequel series. This is what he said to that:
I would be open to anything that Dan and David wanted to do - about Game of Thrones, or any subject matter. It really would depend fully on what they wanted to do. I think you're right, there's enormous storytelling to be mined in a prequel, if George and Dan and David decide they want to tackle that. At this point, all the focus is on the next few years of the show. We haven't had any conversations about that at this point.